On another thread I recently outlined a problem with huge memory issues, which are apparently (though not definitively) caused by an errant plug-in, which has since been (I think, mostly) removed.
 
Now, however, I can't play anything via straight Midi. I have a Roland rack mount synth that I've been using for over ten years. It accepts straighforward Midi through a Midisport 2x2 box connected by USB. All of a sudden now, I hear nothing. My EastWest soft synths play fine, and TTS plays fine. Just this physical external synth.
 
Sonar recognizes notes are being played on my piano, and I can even record them, but there's no sound. When playing notes, I can see the Midi box light up, but the Roland synth does not light up, which normally it does when a note is pressed or released. I have rebooted the synth itself, still no change. This is true for a brand new empty Sonar project, after several reboots.
 
It seems unlikely to be a hardware issue, because I haven't touched either the synth or the Midi box, and they've both been working for years without my touching them, AND just the last few days I've been messing with plug-in manager and such to try and get to the bottom of my memory leak and plug-in issues. I can't think how a plug-in would affect my vanilla physical Midi output, but I don't really understand all this stuff well, so perhaps messing with all that wrecked something, and I need to reset something or re-scan something or something else easy and simple like that? Please?
 
Here is a video demonstrating it. There's no sound, but trust me.... there's no sound
http://imgur.com/a/2C1Sv
 

In the immortal words of Herman Cain, "I don't have facts to back this up" but...I see quite a few issues with the MIDIsport crop up in this and other forums. From what I understand there are drivers, but it's also class-compliant. There may be some issues with Windows updates. I suggest uninstalling the drivers, and plug directly into the computer (using a different USB cable "just in case"). It may work right out of the box. If not, check Device Manager and if the driver shows a warning, uninstall it and re-install the M-Audio drivers. It seems SONAR is doing what it's supposed to do and so is the Roland, so I'd look in what connects the two - the operating system, USB, the cable, and the interface drivers.

Thanks, I'll look into that today.
 
Possibly related, is that I have this big project with 20+ EWQL instruments, about 17 minutes long. Since I've been having this problem, and the memory thing from the other thread, this project seems unstable. Sometimes the audio freaks out or Sonar crashes altogether. Through all this memory trouble I fear the project has become corrupt, and I'm wondering if I might want to start fresh with a new project, copying all the tracks over painstakingly.
 
Is there any best practice step by step documentation anywhere on how I should go about this? I have all these tracks in folders, the instruments, the note events themselves, some buses, a few effects, tempo changes, dozens of markers, track colors, whatever else. The idea of copying this into a clean project piece by piece sounds daunting and dangerous. I'm thinking someone may have written up a paper on how best to go about it.

Do a "Save as copy" and see if that helps. Next, save as a bundle and then unpack into a new project. 
 
However, I think a very likely problem is that 8 GB of memory is simply not enough these days, especially for big projects.

I would test the external MIDI with a fresh blank test project with one MIDI track.  See if the Roland responds to that.
[Edit] Just watched the video you posted.  It appears that you already did this.  Look in your MIDI device settings and make sure that your MIDI outputs are still pointing to the physical ports with the MIDI cables.
 
Then if that still fails, I would proceed with Craig's troubleshooting suggestion in his first reply. 
 
I have a MIDISport and Roland gear, but I haven't connected it since Windows XP, so I'm no help there with Windows 10.  But maybe I will learn something here if I ever decide to plug in my Roland.

Thank you! I will try that.
 
In the meantime, I've been thinking that I barely use that Roland synth much anymore. I asked myself what I really need from it, and I use its basses a lot, though TTS and SI are as good to my ears. I use various real-world instruments not found in EWQL, such as accordion and harmonica, though again, TTS covers that base as well. Then I have a World Instruments add-on card that I'd be sad to see go, but I rarely use it either. So I'm thinking after all these years it's time to bid farewell and go 100% soft synths finally.
 
But not before I try the things mentioned above. :-)

I have found that the AIR Xpand!2 VST instrument has many good real instrument sounds.  There are 2500+ presets included in the library, which is fairly light at 1.5GB.  It loads up quick, is 4-part multitimbral, so good for quick combinations or layers.
 
The best thing is it occasionally goes on sale for $1.  Not to be passed up at that price.  It makes for a decent soft rompler.
 
http://www.airmusictech.c./product/xpand2#features

I just had a similar problem.
Came out of nowhere.
 
I noticed that on the midi send led in the system tray the red light was on solid.  Midi receive would wink whenever I hit a key on my Komplete Kontrol S49.  But no sound.  (virtual key board worked fine)
 
I ended up deleting the ACT controller settings and then re-adding and then midi went back to the way it should.  The fact that the midi send was pinned from the ACT controller settings tells me it was something in Sonar got hosed up.

Update, new information:
 
Following Craig's suggestion, I tried Save As Copy... no difference. However, I found I could repro the crash reliably by adding a new TTS synth and assigning an instrument to Channel 1. Sonar reports a failure and I need to shut it down. At this point, the "Systen and compressed memory" process in Task Manager is up around 3 Gb, something I thought was due to this plug-in I'd added.
 
Then I tried saving as a bundle and unpacking the bundle to a new project, and now things seem a lot more stable. The Task Manager shows Sonar as the biggest consumer of memory, and the System process doesn't spike when I add the TTS instrument, hovering down around 200 Mb. I'll report back if this seeming stability is a mirage or if it holds.
 
 

I have a midisport 4x4 that screwed up with a recent win 10 update. I renamed the TTSSeq file and let SPlat build a new one when it restarted. It's kind of a pain to redo all the friendly names but It fixed the problem. I did the Aud.ini While I was at it. YMMV.