Your preview bar is too short. Take your head and slide it all the way to the start of the comp and hit B (beginning) on the keyboard. Then slide your head to the end of the comp and hit N (end). The hit spacebar to preview. problem solved. Somehow your work area bar got reduced to zero frames. You can also put your head exact at the beginning of what you want to preview and hit B and then move it to the end of what you want to preview and hit N. You do not need the entire comp. Happy editing.