If it cranks over easily, normally, then I would suspect the spark, but if it won't spin over, somebody got something too tight. Did you check the oil? How about the mechanic, did he oil up the new piston? How about the fuel? Fuel pump? Choke stuck on, was the timing set? Does it have an overhead cam? Check the belt? Check the timing again. Then go to the coils. Run/kill button, kick stand switch, is it stuck in gear? Check this stuff out.