The pulley won't turn at all? Is the clutch engaged or disengaged? Belt on or off?
With the belt on, you won't be able to turn the pulley by hand, because you're trying to turn the engine crankshaft by hand. And, assuming you're not built like The Rock, you aren't going to be able to do that.
With the belt OFF and the clutch DISENGAGED (so there's an air gap between the outer clutch plate and the pulley), the outer clutch plate should spin with some resistance. Here, you're actually turning the shaft inside the compressor, so you'll feel the resistance of that, but you should still be able to turn it by hand. If you can't, the compressor is seized. This can be for a variety of reasons, including low/no freon, not enough compressor oil in the freon, or a seized internal compressor bearing. Adding freon *might* fix the problem, but most likely the compressor is hosed.
With the belt OFF and the clutch DISENGAGED, the pulley should spin freely on the compressor shaft. You should feel little or no resistance to spinning the pulley. If the pulley doesn't spin freely, either the clutch is engaged or the bearing inside the pulley is seized (so you're turning the compressor shaft rather than just the pulley). If the pulley won't spin *at all*, then the bearing inside the pulley is seized AND the compressor is seized. Most likely, the pulley bearing has been seized for a while, which means the compressor has been run 100% of the time regardless of clutch engagement/disengagement, and the compressor got worn out as a result.
Hope that helps.
-Bob