Thanks to everybody who's trying to help.  I appreciate you all!
 
My problem is very much like TimV describes.  Everything is going fine, then when I stop the project and hit play there's just no sound.  I'll keep poking around and see if I can find answer. 
 
Again, thanks to you all.

Gotta say, I have seen this happen occasionally, too.

When you say you see the meters moving, which meters are moving?
 
Track > Bus > Master > Mains
 
If there's activity on all of them then the problem is external to SONAR, most probably with your interface or driver.

Bristol_Jonesey
When you say you see the meters moving, which meters are moving?
 
Track > Bus > Master > Mains
 
If there's activity on all of them then the problem is external to SONAR, most probably with your interface or driver.

For me, it's at least the track and master. It didn't occur to me check the main. Next time it happens I'll look at that.
 
I've been assuming that it's external to SONAR, but it's interesting that it hasn't happened anywhere else. I also wonder why closing and reopening the project makes it work again, at least temporarily. It's not even necessary to close and reopen SONAR, just the project. Also, so far this particular problem (meter activity with no sound) only seems to be happening with one project.

TimV is describing perfectly what I'm experiencing. 
 
As and addendum, tonight I opened a project that simply refused to make a sound.  I opened and closed it several times, and still nothing.  Then I exported the audio to see what would happen and it exported perfectly.  Does that give anyone a clue as to what could be happening?
 
Again, thanks to all of you!!

Same question:
 
When you say you see the meters moving, which meters are moving?
 
Track > Bus > Master > Mains
 
If there's activity on all of them then the problem is external to SONAR, most probably with your interface or driver.

Bristol_Jonesy:
 
Track > Bus > Master > Main meters all appear to be moving.
 
Thanks for your help!

Which strongly suggests it's your interface > monitoring.
 
Check your interface and monitors for loose connections - sometimes rebooting can sort of "bridge" a slightly loose connection - it happens with the surround sound on my cinema system all the time. Jack the volume up or switch it on/off and the sound in the rear speakers comes back.
 
Check your USB cable - swap it out if possible with a known to work, good quality one.
 
Also see if there's any setting on your interface or the driver software which might be contributing.

Bristol_Jonesey
Which strongly suggests it's your interface > monitoring.
 
it happens with the surround sound on my cinema system all the time. Jack the volume up or switch it on/off and the sound in the rear speakers comes back.

 
Same here. My Focusrite 18I8 is hooked up to a TC Electronics Leveling Pilot.
Every time I open Sonar I need to crank it up just a notch because if its too low, no sound will come through my speakers.

Many thanks everybody.  Will do!!