Could be seized bearings. But what helps is to describe the setup, e.g. what type of rear hub? Is it a freewheel? Is it a freehub? Is it an internally geared hub? Shifters/Rear Derailleur? Number of speeds? Can you back pedal freely (not just rotate the wheel)?

If the bearings are seized, then we need to know where/which set, and then check to see if they can be repaired and what else might be broken because usually, seized bearings were caused by some other issue to begin with.