i had a key fail on my karts jack shaft a decent way away from my house. it happened while i was over some sand so when i floored it and i didn't go fast, i figured the tire was spinning, then i lost the momentum i had, came to a halt, floored it again, nothing happened.... i tried putting some wood in the key way to limp back home (after searching for the key of course), after a few failed tries, one stick held, i got going, then it snapped, the engine revved up again, and i came to a stop. some of the time while looking for sticks the engine was also still idling. eventually i gave up and got to work pushing it up and down a bunch of hills. eventually, some sand got thrown up into the sprocket (which is free spinning on the js with every step i push the kart) that sand did not help at all. about 2 weeks later i forgot it was broken, took the kart out, started it up, revved it to warm it up, stepped on the gas engine roared, no movement, then i remembered what happened..... a while later i went to go inspect the damage to see what had to be bought/fixed/replaced, and found the sprocket had seized to the js, i have also noticed a big drop in speed, i think the sprocket is slipping.
so that's my little story, exactly what could have happened to you, i really need to bu some lock-tite for my set screws when i re-build my kart.