Ok, seriously pissed off

What's your sample rate set to in your project? Sometimes Windows can get flaky when switching from 44.1 to 48 when going from sounds played in, say, Windows Media Player and then back to SONAR. If you set SONAR to your system sample rate (or vice versa), does it still happen?
 

I agree with Tim. 'about a whole step' has to be sample rate changing from 48 to 44.1 - probably due to some multimedia app changing the rate in between SONAR sessions. But there must also be some miscommunication between SONAR and the driver about what the sample rate is or SONAR should be able to correct it. A quick search did not find a Win10 driver for the C600. If the Win8 driver hasn't been certified to work under Win10, anything is possible.

When did all this start? Try to figure if anything changed
If you know the approximate date
Try restoring your computer back before that date
Couldn't hurt
Checked for malware?

If it happens again (fingers crossed it doesn't, of course), try bouncing down the project to a file and see if that's at the right pitch. If that's different as well, then I'd start to look at controller issues like was mentioned above.
 
Are you running any Windows system sounds? Boot up? Errors, etc.? Try disabling those.
 
Have you got a webcam that has its own audio driver? A lot do these days, that that could be stealing the driver focus. Go into your Device Manager and see if you can disable every audio device except for your recording gear.
 
And yeah, frustrating as hell isn't it? I've had to re-insert the keys a few times on my computer keyboard from getting angry when everything has gone south after a very long session! Perhaps I need some anger management classes or something? HAHA! I don't think anyone here could hold an angry post against you - we've all been there. 

In the event viewer are there any non-note events? It is possible to have the pitch/mod wheels drift from zero (physically) which can be recorded by SONAR.

Thanks for all of your help.  I ended up just going for Platinum.  This was turning into a nightmare and I just don't have time for this crap. 

No and I am beyond angry right now.  I called Cakewalk today about X2 (before I decided to get Platinum) and the tech guy says, "we don't support X2 anymore, but I'll send you some suggestions in your e-mail, you'll get it in five minutes."  Nothing.  Now, the same f'ing thing is happening and I have NO help. 
 
I guess I'm moving to Cubase if I don't figure this out.  Maybe they'll actually help me with their products.  I would hope....

Check the pitch of the guitar audio file in another player. possibly there was a clocking problem while it was being recorded and the file header is saying it's 44.1 when the interface clock was actually running at 48.

You guys are awesome!  I'll reply to you all more directly later, I have to go to work.  One thing that might help:  when I open the standalone version of Rapture Session (was just testing out the pitch), the onscreen keyboard says it's the correct note, but it's not.  It's a whole step off.  So, I go into the edit function of the controller, make it go down two half steps so it's the right note, power the unit off, turn it back on, open Rapture again.... back to being the wrong note. 
 
Lord Tim, I'm going to take you up on that.... I'll be back later.  Thank you all once again!

There are so many reasons why this could be happening, but I'd bet none of them is due to something happening within SONAR. SONAR can only do what it's told to do. Something is feeding it bogus information. There's no guarantee another DAW would solve your problem.
 
I suggest borrowing a different interface and controller to see what happens. Also, if the controller has some sort of reset function, use that.
 
If the problem is outside of SONAR it's not surprising that support can't help. If it is inside of SONAR, it seems virtually impossible that no one else has experienced this problem.

I second installing MidiOX and taking a look at what the MIDI data passing in and out of Sonar and the controller actually is. Spurious pitch-bend data might well cause the problem you're describing, and I've certainly come across keyboard controllers that didn't accurately centre their pitch wheels but would once in a while trickle out pitch-bend instructions.

I might be wrong (wouldn't be the first time) but from your description at least part of the problem seems to be MIDI related.

First of all, I love you guys.  You're all awesome.  Secondly, here's a little update:  I called Cakewalk today, spoke to another tech guy.  Awesome dude, I don't know his name.  He was kind of stumped, himself, considering the sampling rates matched.  He was VERY skeptical that it was Sonar, as were all of you guys.  He asked if I had changed USB ports for my interface - I had done that because I had transported the interface to work with me (guitar instructor) and plugged it into a different computer. 
 
After I spoke to him (I was driving, so I wasn't near my computer), I called my wife to check something.  Just by accident, she unplugged my interface's USB, then put it back into a DIFFERENT USB port.  I get home, laptop is already on from when she had it on earlier.  Keyboard is in tune!  YAY!  Not time for celebration.  I power off.... PRAY.... LOL... computer comes on.... fire up controller..... fire up Rapture.... PRAY LOL ..... choose "grand concert piano" ..... PRAY MORE LOL..... I hit a low E...... SOUNDS RIGHT!  Get out my phone.  DREAM THEATER.  PULL ME UNDER.  Perfect way to check it.  FIRST NOTE.  BOOM.  DEAD NUTS. 
 
Now, time to see what happens in projects.  Big test.  If you guys don't mind checking this thread for another day or two?  I'm going to give the final verdict hopefully by tomorrow.  Please forgive me for not replying to everyone specifically - I DID read ALL the posts and I truly appreciate all the suggestions.  It might be fixed.  If it isn't, I will start going through all the things people have suggested.  Thanks .... thus far.  PRAYING LOL....

Another update..... recorded a quick project with my guitar (GR5) and Rapture using my MIDI controller.  Everything was perfect.  Saved it, reopened.  Perfect.  Rebooted.  Opened this project, about two half steps too HIGH.  Yes, it was higher.  WTF
 
Pissed.  Unplugged interface, uninstalled interface driver.  Reinstalled interface driver.  Boom.  Perfect.  Restarted.  BOOM.  Perfect.  Hoping it stays that way. 
 
Conclusion:  it's definitely not the MIDI controller.  It's 100% the interface.  I don't know why it's freaking out, but that's what it was.  Probably going to pick up another one at some point because I can't take the uncertainty of "maybe it'll work today, maybe it'll make my life a living hell for like four days until I figure WTF is wrong."  (I do a lot of recording LOL)
 
EDIT:
 
By the way, on a sidenote, although related to this.... NOT happy with M-Audio.  Bought this in late 2012 in good faith because of reliability and performance of Keystation 88 controller.  A year or so later, M-Audio sells a bunch of products to Avid.  Bye bye support for C-600.  M-Audio sold it, Avid doesn't care.  Product completely in "limbo" and probably forever.  Too bad, awesome interface, would hate to switch to another one.  Oh well.  Thanks everyone. 
 

Had two M-Audio interfaces. Best things I ever got rid of. Always thought it was me, but it was seemingly never ending driver problems, and I can't imagine the C600 would ever talk well with Win10. Give it the ol heave ho..

There was a time M-Audio made good products and supported them with frequent driver updates.

That time was several years ago.