Had a new problem yesterday. A project I've been working on for a long time stopped responding to Play. No response to the space bar, no response to clicking the Play button.
 
I had only opened the project to listen to it, i.e. I hadn't made any changes at all.
 
Tried arming a track and pressing Record; still no transport. Toggling the Pause button did nothing.
 
I loaded several other projects and they all played normally. I copied and pasted everything from the problem project into a new file, and it played normally. Of course, I lost all my routing, effects and soft synths.
 
Has anybody seen this problem?

Did you try running the project in safe mode? It may be a problem with only one plug-in. To start a project in safe mode hold the SHIFT key while loading the project.

Thanks scook, I'll give that a try.
 
It seems like a long shot since the file played fine the day before and nothing was changed, but it's worth a shot.

I restored thru project folder from the Recycle Bin in order to try it in safe mode. Just to confirm the problem, I loaded it normally first.
 
It played fine.
 
I really don't like it when problems mysteriously disappear. I've learned nothing and can't know if it will happen again.

OK, the problem came back. And I found a sort of solution.
 
I opened the project to work on it and transport didn't work.
 
My default audio settings are 48kHz / 32 bit. This particular project was created by a collaborator who always uses 96/32. 
 
I closed the project changed my default to 96/32 and re-opened the project, and it worked fine.
 
Not really satisfied with this, because I don't understand it.

What interface are you using that produces 32bit analog to digital conversion?

I'm using an RME UFX, as indicated in my signature. Of course, it does AD/DA at 24 bits, but the file bit depth in Settings is at 32 bits.

Check the master bus output. It may be set to "None".

Make sure the timing masters are set correctly in Preferences > Audio > Driver Settings
 

Thanks kev999. The problem isn't no sound, it's not transport. Play (neither the space bar nor the button) doesn't do anything.  And the Master output is my interface's main outputs.
 
scook, thanks, the timing masters are set correctly.