I was riding in forest when suddenly the revs went up and the bike stopped moving. I thought of snapped chain or something, but I was wrong. 1st, 2nd, 4th and 5th gears work fine and there is no 3rd. It just like in neutral, the bike does not move. There was no abnormal noises coming from the engine when it worked, there is no noise now and there wasnt any when I lost the gear. It just went.

I took the engine out of the frame and I'll try to split it as soon as I can. Does anybody have any ideas what might be wrong with my bikes transmission or do I have to let it be a surprise?

I will take pictures when splitting the engine...