Engine runs but wheels do not move
This could be a few things:
1) Your clutch plates may have worn out so that even when the bike is in gear, it will just rev, but no power will be transmitted to the drive shaft (smaller front chain sprocket is the driving gear, larger rear sprocket on the back wheel is the driven gear)
2) The front chain drive sprocket has come off under the cover on the motor - if the chain is really slack, have a look.
3) The front chain drive sprocket SHAFT has sheared. (basically, meaning that the shaft has broken into two parts as it couldn't handle the drive power (torque) being delivered through it to the chain drive) With the motor on, listen for grinding noises internally
when you rev, from the vicinity of the front chain drive cover. If its rough and grindy, the shaft may be sheared. Then with the motor off, take the cover off and leverage the small chain drive sprocket away from the motor with a screwdriver, and see if the shaft can just be pulled out of its sealed opening. Unfortunately with these bikes, the metals used in drive shafts and motors are not always heat-treated or hardened to the standard they should be, so shearing and premature failure can happen.
4) The gearbox or something in it has blown, unfortunately for the same reason above, quality related.
Its fair to say that these great little bikes are here for a good time, but not for a long time.