@themailgirl you do want to check to make sure that the transmission releases may have been engaged. Those are the levers located at the back of the mower, just under the hitch, that need to be released so the mower can be moved without the engine running. They are basically two rods that sit side-by-side with loops on their ends for handholds and it is possible for them to catch on something and pull out, disengaging the transmissions. In that case, just lift the levers up and push them in.
Of course it is also possible that the rods have hit something underneath the unit and were damaged so that the transmissions are disengaged even with the levers pushed in. So check for any damage that may prevent the levers from moving. Even so you said that the belts are tight, you do want to make sure that your drive belt actually moves and does not slip or does not move at all.
Of course it could be the hydrostatic drive not having enough fluids etc, but for that I would need to know which drive this is. Craftsman usually use Hydrogear transaxles and trying to determine which one. See if you can take some pictures of your rear axle so we can try to identify it. Use this guide Adding images to an existing question for that the top fan/pullet location etc. is usually a good identifier.