Why won't the sound work on only one of my projects in FL Sudio 8, but works fine for all my other projects?

I have FL Studio 8, and I composed one of my beats today and the sound all of a sudden stopped working. At first I could still record my beat to a wav file, just not be able to hear it. Now i can't even record sound at all. I eventually realized that the problem only existed in that project, and all the other projects were just fine. I have no clue on what to do with this.