I have a Specialized Allez Elite road bike, purchased brand new in June this year. So far I've covered around 300 miles in total on it.
During my ride yesterday, as I was going up a slight hill, I switched to the smaller front chain ring. As I did this, I heard a sickening crunch, pedals jammed, looked down to find one of the gear changing parts is jammed into the back chain rings and chain has come off. Luckily I managed to get onto the footpath and dismount without any problems.
I don't know much about bikes so have provided an image of the damage:
Can anyone advise on what parts will need replacing? Also, how can I prevent this from happening again?