any plugs on the track? is it a synth output or an audio track output??

HA HA! Problem solved. I am SUCH an idiot. It's Guitar Rig's RIDICULOUS instance on opening with only the left channel active as default. Forgetting this, I placed an instance of Guitar Rig on the master bus in order to use their Solid Bus Comp, without activating the right channel as well. 

Sorry for perplexing everyone. 

PHEW. If I was a smoker I would have definitely gotten through a whole pack during that scare 

Glad to hear that you figured it out....can be very frustrating for sure...!

Nothing worse than thinking you've corrupted a project beyond repair. Happened to me with Pro Tools once and it was not salvageable. 

I would like to make a correction to my above statement about Guitar Rig. It's not so much that it's ridiculous that it opens in mono by default, it's that you have to open an instance of the freaking thing to use any NI effects in the first place. Hopefully with their recent upgrade of the Vintage Compressors to standalone versions, they might get around to doing the same with everything else as well. 

weird i've never had guitar rig do that. always opens in stereo.  there's kore player and the reaktor fx too if you don't dig gr.  i'm happy they solved the cpu issues between 3 and 5 for one.

Are you sure? Because I think there is a bug whereby the meters at the top show a stereo input even when it's in mono. When you open it, only the "L" is activated (the L and R next to the input meter). You have to activate the "R" as well to make it stereo. I've heard quite a few people complain that it opens in mono. 

ah, that's because in gr i have options/default plugin input configuration set to stereo. 

sharke, Super glad you solved this and no apologies needed.  This was an interesting thread.  It is always helpful for all of us to think through such problems.  Who knows?  One of the wrong solutions above may be the right one for someone else.