Silliarse

While pottering around on the dunes with my son, my bike, a gas gas 125 has decided to kill itself. While in top gear (and thankfully on a straight bit of beach) the throttle seemed to jam wide open, with no response from throttle, clutch or kill switch (once I`d wrestled it to a halt.) After continuing to scream it then rapidly died. And surprise surprise it isn`t keen on starting, just sounds horribly rattley on the kick start. So my questions to you clever people are What caused this,what`s the damage likely to be and once rebuilt how do I prevent it happening again? I had just filled it with fresh fuel containing 63ml of putoline. Thanks, Jack