My go cart motor wont turn over. HELP!!!!!?

i got a go kart at a garage sale for $10 guy said he just had the motor rebuilt but it needed some carburetor work. we adjusted a few things and got the carb working, started it and it ran for about a minute, and then died. tried to restart it and the string wouldn't pull over 5 inches took off the cover and the string was fine the problem was the motor under the string cover will not move at all. it did not make any noises or anything before this happened. it just quit and the motor hasn't turned over all day. can anybody help me? hopefully soon/ quick hoping to get working on it again tomorrow. thanks a lot.