Throttle revs but doesn't catch for moped to move?

I was on my Piaggio Vespa ET2 coming home and I felt a feint knock and on revving the throttle, it wouldn't get into automatic gear and glided, with the engine running, to a halt. it revs on the throttel but the moped doesn't move. I have looked under the seat but see nothing wrong. What do you think is wrong and how may I mend it? I much appreciate your help whomever answers this. Charlie.