Had this happen twice now. the control gets rotated full clockwise by mistake, IMMEDIATELY starting a clean cycle. The indicator led starts flashing, and you want hot water NOW......no.
I my mind this is a major problem with this water heater. The ONLY way to reset assumes you have plenty of water available.
With the control set to normal operation (full ccw) run a hot water faucet for up to 15 minutes. When the light stops flashing, go outside and cycle the power off 30 seconds then-on.