Sounds like a "crank stuffer" bolt backed out and has jammed the rod.
The crank stuffers are plastic plugs that bolt into the crank weights to increase volume in the case area.
There are 6 allen bolts that hold these plastic inserts in place. They have been know to back out and jam up the rod.
It has happned to my bike, same thing, I was warming it up one day, hit the kill button, then wanted to start it again, bam, locked up.
I was able to pull the topend off, I pulled the motor out of the frame so I could work on it on the bench.
I was able to "fish out " the little bolt, I had to cut down a " L" shaped allen wrench so it would fit between the weights. A drop of loctite and threaded it back in.
The other way is to split the cases pull the crank, have the crank seperated and install the bolts.
http://partsfinder.onlinemicrofiche...e=13&make=gasgasdb&a=5&b=22&c=0&d=-CRANKSHAFT
Make sure the rod is not damaged.
Good Luck