If it's wedged in there really good between the cassette and the chainstays (frame) you'll need to loosen the wheel like you were going to take it off to get the chain free... and then pull it back over one of the smaller sprockets. Make sure your derailleur is lined up with the sprocket you put the chain back on before you try to ride again (just eyeball it from above - if you're off by a gear either way nothing catastrophic should happen).

Secondly, how/why did it come off to begin with? Were you shifting when it happened or did it go off on it's own? It might be worth stopping by your LBS so they can check the cable to make sure it's adjusted fine and nothing slipped.