There are a few possibilities here. When you rotate the back tire does the engine turn over? i.e. a popping sound. If so, then your butterfly nut probably needs tightening. To do this, open the large plate on the side opposite the clutch arm and remove the small silver screw, then rotate the star-shaped butterfly nut. You'll probably have to use a hammer and nail set to do this, as the nut is usually pretty tight.
If the back tire does not turn the engine over, and you just can't make it go at all, then your chain has probably skipped a tooth within the clutch itself. This is a pain in the butt, but not the end of the world. If you remove the clutch arm plate you'll see it right away. I typically use a ratchet with a 1/2" nut and put a steel pipe on the end of the ratchet for leverage. This happened often to me when the chain derailed.
A third possibility is that the clutch bearing fell out. It's a small silver bb about the size of buckshot that rests behind the rod. Mine fell out and I didn't know it and was quite perplexed. You can usually tell if it's missing because the rod will go in much further than it usually does.
Hope that helps in some way.