One Side of Audio Missing

One file/project I am working on is suddenly missing the right side.
 
• Other projects are fine
• Speakers and headphones are all working properly.  Tested with non-Sonar audio and with other Sonar files.
• Master and individual track meters show audio playing on both sides.
 
What the heck?!?  It is as though I accidentally hit some key that turned off audio to the right side.

Not much info to go on here...

What steps have you tried?

My first guess would be to check the output of the Master buss and make sure it is routed to a stereo pair.  Even if it were, I'd switch it to another output and then switch it back.
My second guess would be to check your plugins.  Certain plugins will cause this behavior.

My first question would be, what was going on when the project stopped working?
It is much easier to troubleshoot when there are at least a few details given.
Such as:
The project was working fine. I inserted suchnsuch plugin, saved the project, then it stopped working.  I shut down Sonar, toggled my audio interface off,then on, restarted Sonar...

If I fiddled with this for more than 10 minutes and still couldn't narrow it down and get it working, I would copy all of the tracks to a new project.