I think it sounds like it came off the rings (of the crankset) and not the cassette sprockets, since you said it happened with the left shifter. It's not uncommon, though it usually doesn't get really jammed in anywhere. Try pedaling backwards and shifting again in the front. That may free it without getting off the bike. Otherwise, you'll need to get off and pull the chain manually (from the bottom) and place it on the small ring. Then hand pedal to get it engaged.