I can't make heads or tails of this one, but it's very consistent so hopefully I'm just missing something obvious... Using Sonar Platinum and Windows 10 here.
 
I made a new project that acts like it's breaking a connection somewhere between midi data and playback. When I open the file and hit play I can hear everything just fine. If I let it play until the end, I can stop and play it again just fine. But if I stop playback anywhere in the middle, there is no more sound to be had. When I hit play again I can see the lights going in the meters for the midi tracks, but nothing in the soft synth tracks or main output. No sound will play until I close and reopen the file. If I open up Kontact and try to click around on the keyboard I also get no sound there. The metronome, however, still makes noise.
 
Things I have tried (and which had zero effect):
* resetting midi and audio
* stopping the audio engine (why not try?)
* purging and reloading all samples
* increasing playback buffers
* updating to latest version of Sonar
* unchecking that pesky "allow applications to take exclusive control of this device" box in the properties manager for my audio interface
 
Other info that might be pertinent:
* This is not my first project in Sonar Platinum, but it is my first project on Windows 10.
* This is my first project with this sample library (LASS)
* Older projects do not seem to be affected
* I have a bunch of samples loaded but few in use so far
* I am nowhere near heavy CPU load, and my ram is about 50% (~9GB out of 16)
* Audio Interface = Roland Quad Capture
* East West's Play engine is loaded, as are multiple instances of Kontact and one of ENGINE
 
thank you very much for any help or thoughts you might have! 

Welcome to the forum!
 
Try unchecking "zero controllers when play stops" under preferences>project>midi.
 
Note that this option will only be visible when a project is loaded. It creates exactly the problem you are describing in a few Kontakt libraries that use non standard cc assignments to vary dynamics.

Thank you thank you!

That has done the trick. I read up on that option in the documentation after seeing your post and it *seems* like the kind of thing that should have the opposite effect, but hey, it works. Maybe it has to do with how the library handles "zero".
 
And thank you for the welcome :) People here seem very kind.

violacompost
 Maybe it has to do with how the library handles "zero".

Most times I have encountered this problem that is exactly the issue. More specifically how the library handles a zero value on modulation control. Quite a few string and brass libraries use the mod wheel for expression changes that incorporate volume, and a zero value gives no sound.
 
Very occasionally, if you are working on a project with lots of piano/sustain pedal you may find that you need to recheck zero all controllers to avoid open sustain control changes when jumping around the timeline. In this instance, if you encounter the zero controller losing audio issue, as a workaround, with the zeroed track selected,  try moving the mod wheel after you have stopped playback to restore sound. It is a bit tedious, but most times it is the culprit, and this workaround will get you through those rare occasions where you need to have zero controllers checked.  
 
It would be great if you could choose which controllers were zeroed!
 
Glad you got it sorted:)

Awesome that you got it worked out so soon after posting the issue. This is the best and most helpful forum I've ever been a part of. Just a note - You may want to edit the subject line of your post to reflect that your problem has been solved. You'll occasionally see a post with <SOLVED> in the header, something like that would be helpful to forum readers. Welcome to the forum!

mrpippy2
You may want to edit the subject line of your post to reflect that your problem has been solved. You'll occasionally see a post with <SOLVED> in the header, something like that would be helpful to forum readers. Welcome to the forum!

Thanks for the tip. I figured marking it "answered" was good, but this makes it a little more obvious.