Not sure if this started with X2 or whether it happened with X1 as well (can't remember), but very occasionally, Sonar will not load for me after I've quit out of it. I sometimes restart it if things start going a bit iffy. And even though I'm pressing the Sonar icon in the task bar to reload it, nothing happens. The icon stays highlighted as it would if it had registered the request, but no Sonar appears. I have to reboot. Very weird. 

Have you tried turning power cycling your audio interface? 

This started happening to me too. I have to re-start pc to get it working again. I haven't tried power cycling the interface...I'll try that next time it happens.

What is power cycling the interface when it's at home? 

I have dealt with the many times. Something similar also happens, when I close out Sonar and shut off the computer, i get a crash before the computer actually powers down. And the fix I use is simple. Go to task manager and look to see if Sonar.exe is still in use. select it, end task, it should shut down and then you should be able to click the icon and it should open again. I have done this many many times.  Seems like sometimes it takes the computer a while to "unload" sonar. so if i shut the computer while that file is still in use, i get a crash error and the next time I boot and open sonar, my most recent project is not the default "open project".  And I have also closed sonar and then tried to open it again. and nothing. every time, the sonar.exe was still in use.

Unfortunately that's not the case with me TraceyStudios - first thing I do is to check the task manager and there's no sonar.exe there.

Check your Task Manager 'services' to see if Bitbridge.exe is still active. Sometimes 1 or more; 32 bit plug(s), do not dismount after Sonar is close/exit.

i've experienced this since I can remember...I've been opening  Sonar/Cakewalk almost everyday for 20 years.  I did not do Sonar 1,  but I'm pretty sure it happened in Sonar 2.  It might be something to do with my soundcard 'cause it seems when it happens other Audio apps do not open or work either.   Just need to do a reboot.  XP and earlier versions needed to reboot occasionally just because.   

I think it's related to the audio drivers not being released. It used to be more common when I was using an internal ESI Juli@ card but it still happened occasionally on my old DAW with the Quad Capture. Its too early to say if the problem persists on my new DAW because I have not used it much yet.

Sharke . . . happens to me occasionally as well . . . when I do something Sonar doesn't like, it hangs, so I force close it down, but Sonar will not re-launch, I've waited, gone for coffee, even beers, but I always lose . . . just reboot.

I believe this is the soundcard holding onto something in Sonar in the backgound, somewhere, and I recall other threads discussing that some M-Audio cards are notorious for this, though it's probably not just M-Audio cards.

Fortunately, I've  never had it corrupt a project or lose data, reboot, and I'm running again.