Song will not play

Just spent 2 days working on a song. Suddenly i have a problem. it opens, shows all of the instruments and tracks, but when i press play there is no sound. The bar moves as if playing but no sound. If i try to press any of the tracks to edit them it forces the program to close completely. This is extremely frustrating after all of my hard work. Does anybody know what may cause this problem and how to fix it? I really need to save this one! Please help!