Diagnosis
The thermal overload protector in the motor is tripping instantly. This is usually because the pump is seized or locked up, putting too much load on the motor the moment it starts.
How to Fix It
Unplug the machine. The first thing to try is manually turning the pump to see if it's frozen. Locate the fan cover on the end of the motor opposite the pump. There's a plastic fan behind that cover. Try to spin that fan blade by hand. If it won't budge, the pump is seized. Sometimes you can free it by removing the spray tip and pulling the trigger to relieve any residual pressure, then trying to spin the fan again. If it spins freely, plug it back in and try starting it with the trigger already pulled.
If That Didn't Work
If the fan spins but the machine still trips off, the issue could be in the motor itself or a faulty thermal switch. This is less common, but if the machine was stored with water in the pump and it froze, internal damage is likely.
Heads Up
Let the machine sit unplugged for at least 30 minutes if the thermal overload has tripped. It needs time to cool down and reset before you try any troubleshooting or it won't respond.