Aha! It's very likely that there is enough hair/string wrapped around the brush that it bound the belt.

There should be some screws on the bottom so you can take the cover off and get in there and clean it out. You may find a broken belt in there (or a severely compromised one). You can get the belt model number, hit a hardware store and get a new one to replace it. Then you fit the brush back in, tightened the cover on with the screws, and your vacuum works better than ever, for $10 or less.

(Those belts can REALLY stink when that happens. Blech.)