how can i fix my cr 125r 1997?

yesterday i was riding my cr 125r and i had a small stack and dropped the bike i got back on and started it. it stared as usual then i took off down a straight it wouldn't go into power ban. when i got to the end of the straight it stalled and hasn't started since. i changed the spark plug which did nothing. as i was looking over the bike i noticed i was missing the breather pipe from the transmition and it leaked out a lot. i had also changed the packing in the muffler the day before but i don't think that is related to the problem because for the first hour of the ride it was fine