Is it frozen with the A/C on (i.e. clutch engaged) or when the A/C is off (clutch disengaged)? If it is frozen with A/C off, it might just be the pulley bearing that lets the pulley spin freely when the A/C clutch is not engaged. That can be repaired.
But if it is frozen internally you've probably got a mess on your hands. Some a/c places call it the "black death" from all the gunk that spreads through the system. Need a total flush as MT said. That means diassembling everything and flushing until clear. They make DIY kits for this that have a solvent container that you connect to your air compressor. Has a line with rubber adaptor for inserting into a/c lines and components. Then need new comp, receiver/accumulator, new expansion orifice, o-rings, etc.
Be advised that most a/c shops won't do this w/o replacing the condenser since the tiny parallel passages are extremely difficult to flush and the shop won't be willing to warrant the work.