I have a project that the entire left side of the mix has no sound output.  According to all the settings, everything should be fine. The meters show both sides have output, etc. If I set the master fader to mono I get no output at all. This is just on one song of several by the same band. It had been fine, but opened it up last night, and no output on the left.  All other projects play back fine, so it's not a hardware issue.

Any ideas??????

Check the pan setting on the master fader and check the interleave button.

is there a problem with the connection between your interface and your stereo?

So not a hardware issue then, if all other projects play back ok..

Can you revert to an earlier saved version, and see how that plays? 

If so, then perhaps the newest version had something done & saved without you realizing it.

I say this because I've sometimes changed a setting with the mouse scroll wheel, only to realize later that I didn't even catch it at the time, because I neglected to 'click out' of the VST or something, and back onto the 'main frame'.  

What do you have on the master bus?
 
If you disable all FX (press "E") is there any output?

Thanks for all the replies. Turned out the left fader on the 1/2 pair on the "Mains" screen had been turned down. Not sure how that happened since I NEVER use that screen. Really had me scratching my head!