I was riding along on my CPI SX 50 until the Back Wheel had locked?

I was casually riding along on my cpi sx 50 when the back wheel suddenly locked up and the bike cut out. I tried to kick start the bike but it wouldnt kick over. I bumped started it and it work until it happend again, now this time the bike lost compression and wouldnt bump or kick. I took it into the garage and they changed it back into a 50cc, as i rode off with it, down the road it did it again, this time it still wouldnt kick over. So i bumped it and worked. what could be the problem?