I was changing a bunch of reverb and volume settings, trying to get a better reverb effect, and when I played it back, there was no sound from the left speaker. I checked connections, no problems.  I closed the piece, opened a different one and it played fine. So the problem definitely was caused by the changes I made, detailed below:
 
I set midi learn on my vsti's, so I could control reverb with the Sonar sliders. I zeroed all volume sliders in console view, for vsti's, busses etc. I set all instruments on vsti's to default volumes. I did not adjust any pan settings.
 
I was able to open another version of the piece and save it to the one that I messed up, so all is well. But I'm just really puzzled by what could have happened here. It doesn't make sense. What the heck could have caused this?
 
 

So, the question begs, which reverb plug?

I'm using each synth's native verb, and also sending each synth to Spaces. That way I can adjust each inst. singly, but also put everything in the same space. I didn't make any changes to Spaces or how I send the synths there, just to the sliders (CC 91), as well as the volume changes.

I should also mention that I'm using 8.5 Studio, in case that makes any difference. I plan to upgrade when that becomes possible, but for now that's my best Sonar.