First, you could try dribbling a little 10w-30 or similar oil inside the freewheel while it's still on the wheel. Maybe then, with some brute force and luck, you can get it back to spinning. Lay the wheel flat and notice which part spins, and which is stationary. Get some oil worked down between the there and loosen it up.
After you've wasted 1/2 hour doing that, remove the freewheel and put on a new one.