DIAGNOSIS Water has gotten somewhere it shouldn't, creating a ground fault. The most common culprit is moisture in the on/off switch housing or a damaged power cord.
FIX 1) Unplug the washer and dry EVERYTHING thoroughly—the machine, the cord, the outlet cover. 2) Check the power cord along its entire length for cracks, cuts, or exposed wire. 3) Inspect the switch area on the machine for visible water ingress. Let it sit in a dry, warm place for a day. If it works after drying, you've confirmed the issue. For a more permanent fix, you can carefully open the switch housing, dry the contacts, and seal it with dielectric grease.
IF THAT DIDN'T WORK The GFCI outlet itself could be overly sensitive. Try a different, known-good GFCI outlet on a separate circuit. If it works there, your original outlet may be faulty.
HEADS UP This is a safety feature preventing electrocution. Don't bypass it by using a non-GFCI outlet. If the problem persists after drying, the motor or internal wiring may be compromised and the unit should be serviced.