DIAGNOSIS Residual water has likely frozen and damaged the pump, or it's gummed up with deposits. The pump seals and valves are the most vulnerable parts.
FIX 1) Don't just plug it in and hope. First, check the pump for obvious cracks or damage. 2) Remove the inlet and outlet hoses. 3) Try to manually turn the pump shaft (the center of the pulley) with a wrench. If it turns smoothly, that's good. If it's stuck, you may be able to free it by pouring warm water into the inlet or using a pump saver/anti-freeze fluid designed for pressure washers. 4) If it turns, reconnect everything, ensure water flow, and try starting with the trigger open to flush the system.
IF THAT DIDN'T WORK If the motor runs but no water comes out, or it leaks from the pump seals, the pump internals are damaged from freezing. The pump assembly will need to be replaced.
HEADS UP Proper storage is key. Always run RV/Marine antifreeze through the pump before storing, or use an air compressor to blow all the water out of the pump and hoses. Never store it with water inside.