Same thing happened to mine.The problem is caused when the trimmer is snagged on a wire or too tough a twig. The drive worm continues to turn for a second or so which strips teeth from the worm gear. Now each time the drive worm is opposite the gap where the missing teeth are, the motor spins the drive worm but it is not engaged in the worm gear so nothing happens.(The same thing happens with car starter motors). When you give it a tap this can sometimes move the gear enough to re-engage it so it starts again until the gap comes round. Sorry but you need a replacement/repair to your drive.
Albert, May 2012
