If the chain turns around but does not drive the rear wheel then the problem is with the rear cluster, called a freewheel. Either the internal parts are gummed up so that they no longer move freely in order to drive the wheel or they are broken, the first being more likely. The first thing to try is to remove the freewheel (requires a special tool) and ether submerse it in mineral spirits or run the solvent through the freewheel. Best done at a shop.
