DIAGNOSIS This is classic airlock, especially if it's the first start-up or after storage. The pump can't prime because air is trapped inside.
FIX 1) Make sure your garden hose is connected and water is flowing freely to the washer. 2) Remove the high-pressure hose from the pump outlet. 3) Point the hose into a bucket or safe area. 4) Pull the trigger on the spray wand and turn the machine on for 10-15 seconds. You should see a sputter of air and water. 5) Turn it off, reattach the hose, and try again. The pump should prime and run normally.
IF THAT DIDN'T WORK If it still just hums, the pump might be mechanically seized from sitting with moisture inside. You can try removing the pump's inlet screen and gently turning the pump shaft with pliers to free it. If it won't budge, the pump is likely frozen and needs replacement.
HEADS UP Always run the machine with water flowing for at least two minutes after you're done to clear the pump and prevent freeze-up. Never run the pump dry for more than a few seconds.