Project Won't Play

for some unknown reason, a project i'm working on will suddenly not play.  the audio engine red box is lit and it won't play.  not sure what i did.  past versions of the project with i think the same number of tracks still play fine. 
 
the project consists of 40 tracks, comprised of soft synths and audio, just like previous versions that still play.
 
any ideas?
 
thanks.

tried it - no response.
 
[rebooted computer and it's working; just a fluke i guess]