No sound on playback suddenly.

I had that sort of thing happen a couple of times with X1, and never got to the bottom of it.  I just moved everything to a new project and it was fine.
 
Bob Bone
 

Preferences > Audio > Driver Settings > 
 
Is the Playback Timing Master Set?
 
Sometimes this can get set to NONE depending on how you turn off your equipment.

Is the now time marker moving? If no, did you accidentally enable looping?
 
Also, Make sure your tracks are routed to your master bus and your buses are routed to your master bus and your master bus is routed to your correct main outs.  If the meters are not moving, then its a routing issue, so check these things above.
 
CJ

If you haven't dragged everything over yet to a new project, I am wondering what would happen if you did a 'Save As' for the project, and saved it to a brand new project folder (create the new project folder during the 'Save As' dialog), also checking the Copy All Audio check box, so that all of the audio clips would get copied over as well.
 
That would be pretty easy to do, and maybe it would somehow correct  whatever was goofed up in the project.
 
I would suggest that if you DO do the above, that you give the project file a new name in the process, to match the new folder name, so that there is no confusion with having 2 project files with the same name.
 
Worth a shot, I would think, and not much time lost if it doesn't work.
 
Bob Bone
 

I've had that happen a couple of times and I ended up rebooting my computer to get it working again.

I've had this happen a lot - I don't know what triggers it, and when I dig into settings everything seems to be fine. Some projects seem more susceptible than others, though I have not been able to figure out why.
 
By now you may have already tried this, but I found that closing sonar and re-opening the project solved the issue short-term. Eventually sound for some or all instruments would disappear again, but closing and re-opening would reset it. If you're on a deadline and pulling out your hair, hopefully that will at least get you through to the finish line.

Another thing to try that I forgot about until I was looking at another thread about something unrelated:
 
Unplug any midi controllers, restart, see if the issue persists. There's a chance a midi controller is sending unintended signals and screwing things up for you.

Ok an update.
I go to an any soft synth and click the midi input and change it and then I can play the keyboard and the softsynth plays.
 
After that, when I play the project, no sound and that softsynth no longer plays again until I click on that midi input again and re-select it.
 
Also, still, the guitar parts and vocals which are only audio still do not play. Total silence and no meters when playing back the project.
 
I wonder if there is some kind of reset for the project I can do or something.