So, one particular instrument is suddenly very quiet in the mix.  Nothing I can do will increase the volume  (It's a kontakt instrument and I turned the volume all the way up)  I even put a compressor on it and it didn't increase the volume!  The second weird thing is that Sonar suddenly can't see my nanokontrol 2.  It's not available in preferences, but it is showing installed ok in device manager.

Hi. Have you tried to remove, and add you Nanokontrol again? I do not have this controller, but it might be worth a try.
All the best.

Regarding the volume issue...unintended automation?
 
Don't know what OS you're using but re the nanoKontrol 2, when I had that happen it was the Windows MIDI port limit issue. 

What version of Kontakt? There was a version of Kontakt that would reset the controllers to 0 every time it started.  I can't remember the details.
Max Arwood
 

Anderton
Regarding the volume issue...unintended automation?
 
Don't know what OS you're using but re the nanoKontrol 2, when I had that happen it was the Windows MIDI port limit issue. 

This appears to have been the problem.  Thanks

In general, a compressor will decrease the "volume" of a signal. It can be used to increase the overall saturation of an audio mix, but it does so by keeping the hottest values from clipping while you increase the power of the quieter portions by applying some other gain. The tradeoff is a decrease in the dynamic range.