Everything was working fine.  Now, I can't get the project to playback.  I press the spacebar (which used to start the project playing from where the playhead was located) but now, nothing happens.  I can manually move the playhead, but nothing shows in the player window.  I can save the project (Ctrl S) so I know the keyboard is working.  I can double-click on a clip and bring it up in the window (and run it) but I can't seem to get the project to play.
This never used to happen so I assume I've accidently hit some combination of keys that has "broken" the software.
Any suggestions?