Rear gears don't turn anticlockwise
Well, the freewheel/freehub is frozen, apparently.  Generally nothing to do but replace that component (repair is possible but difficult).

But there are two other possibilities:


Look between the big gear and the hub to make sure there's nothing (eg, a pebble) jammed in there.
The nuts on the axle (including some you can't see) may have somehow become overtightened.  You can try loosening the nuts you can see and see if that improves things, but you might as well take the wheel to your local bike shop since they have the tools to take it apart and know how to adjust it.