The best way to help you is if you can list the manufacturer and the model water heater.
If it's a power vent water heater that plugs in, I would unplug it for 30 seconds and plug it back in and see if it starts. Plumbertom
Turn the power off for ten minutes to the water heater. then turn it back on. I'll be online. Please tell me what happens. Thanks. plumberTom
The water heater has been installed for awhile? It isn't new? PlumberTom. Does your board have a reset button?
Customer You have to reset the board three times. If you have a reset button on the board, push the button, if not turn off the power to the unit. Wait ten minutes. Repeat 3 times. I know it's a pain. Sorry. If this doesn't work after 3 tries you need a new board.
I hope it works!!! Plumbertom