MG ZR Timing belt engine lock up

I have a 2004 MG ZR and i needed the water pump changing, the mechanic also changed the timing belt as it had to come off anyway, this was done in january. The other day i was driving and started to stop to come up to a junction and it shuddered and cut out, to begin with it wouldnt even attemp to turn over and after 15/20 mins it started to attempt to start but couldnt, everytime i turned the key to start it there was a horrible thumping noise from the front and it sounded like my exhuast unless it was just back fireing. When the AA man got there my timing belt wasnt moving and there were tiny shards of metal that had fallen on the inside of the gear things, the timing marks of the top two were both pointed at 2pm. I have a feeling that this is because of the mechanic maybe not doing things up properly and something jumping? or is this something different. i didnt notice any lack in power.