One of two things may have happened from your description.
The chain fell off or the wheel is cocked against the frame. If the pedals spin freely the chain is off. Place the chain around the rear gear and at the top of the front gear and then pedal the bike with the back off the ground. The chain should go back on as you pedal. Stop the back wheel and check the chain for tension. If it moves up and down more than 1" total movement the rear wheel needs to be positioned back for the proper chain tension.
If the rear wheel just doesn't move and the chain is already in place you may have a jammed back wheel because it isn't tight enough at the axle nuts. That requires you to loosen the axle nuts and reposition the wheel straight, with the proper chain tension, and then tighten the axle nuts.
If you don't have the tools you will load the bike in your car and take it to your nearest local bike shop. The fee for fixing this should be nominal. Show them the bike and ask them how much before they work on it.
