Either the inlet valve is faulty or the pressure switch is not controlling the water level. If the machine is still filling with the power off, it will definitely be the fault. If its intermittent, it could be the pressure switch. There is a remote possibility that the electronic controller has failed.
Make sure your water taps are on full, the hoses aren't kinked or bent and if there are any small filter screens to the machine water inlets that they are not blocked with debris. The H20 error is indicating the machines' water level is being interfered with somehow.