Diagnosis
The pump is seized. This happens when it sits with water inside and the internal components corrode or mineral deposits lock it up.
How to Fix It
Unplug the unit. Remove the pump housing end cap. Manually turn the pump shaft with a wrench or socket. Turn it clockwise. It will be stiff. Keep turning until it rotates freely. Reassemble. Before first use after this, pull the trigger to relieve pressure on startup.
If That Didn't Work
If the shaft won't budge, the pump is permanently seized. You need a replacement pump assembly. Part number is usually stamped on the pump housing.
Heads Up
Always run the pressure washer for 30 seconds with the trigger pulled after each use. This releases pressure and drains most water from the pump to prevent this.