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.