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

The throttle has jammed on.

In the event of the throttle jamming on you can pull the cable out of the twist grip control at the handle bars, purely by reefing it out, this will normally release the jammed throttle.
Second option is to cover the exhaust outlet, if an engine can't expel gases it cant run.
Also you could have jammed the brakes on and killed the engine that way.

If you are lucky you have only destroyed the top end and the bottom end is still usable, it needs to be disassembled and checked.

 Minimum would be a top end and cylinder. But as bad as it sounds could also be a rod. In case you missed it, it`s not a race bike. It could of been the cable, the throttle assembly, the slide, carb body or missing the needle jet and or the cable attached to the slide improperly. Good luck. 

Could it not just be, hopefully...... the woodruff key ?

 That it could, but I would just pull the header pipe to have a look,see.

I agree with rotors7. I've seen bikes do that with only result being shearing the woodruff key locating the flywheel. You could be lucky.