you have to look over the rear axle carefully, you must loosen the big nuts to move the axle back, there should be adjusters with a lock nut on both sides the axle mest be clean the area well there should be some litll marks you can use to keep the axle straight. otherwise if it is crooked the chain will come off the sprocket. In extreme cases the chain may arleady be pushed all the way back, More than likely the chain will need to be replaced it has stretched . And as the other answerer wrote, it is always a good idea to get a shop repair manual if you plan to work on your own stuff.
