Broken Cogset-Back Wheel will not turn when pedaling
Your rear freehub or freewheel is damaged and/or needing service.  The mechanism that engages when you pedal forward, then releases to spin/idle when you are not pedaling or pedaling backwards is broken.  Much of the time these are not serviceable, or are damaged beyond service once they are spinning forward freely.  If you have a freehub, you will most likely need to buy a new rear wheel.  If you have an older style freewheel, it will need to be replaced.