When using Garritan IO, I can play notes using either the on-screen interface or my midi keyboard. When I write some notes on the piano roll. I press play and it plays as it should.
 
However, if I change loop points, or toggle loop points on or off, there is no more sound, either from the piano roll, the IO interface, or from my midi keyboard. Undo doesn't help. It also seems to occur if I move notes on the piano roll. Other synths are not affected.
 
I have stopped and restarted the audio engine, but to no avail. The only thing I've found that works is exiting and starting over. If I save the file, restart Sonar and reload the file, there is still no sound for this instrument. 
 
I'm using Sonar X1 build 533 x64. Any suggestions? 
 
Thanks.

As is usual after publicly advertising my ignorance I feel kind of silly. Mr. Holmes over at Garritan explained the issue to me; after edits on the PRV, the GIO plugin resets the mod wheel (volume) to 0. So, if anyone ever runs into this, resetting the CC1 to non-zero solves the problem. 

I wouldn't say that is silly.  I am not sure what the purpose of that feature is and wouldn't have thought that it would happen after editing in PRV.