The low pressure light came on after the pressure fell below the normal range. I have increased the pressure in the system so that the gauge is now within the normal range however the low pressure light is still on and the hot water is not working.

JG, May 2006

My 100e did the same, due to debris/corrosion blocking the inlet to the pressure switch. It needs removing and cleaning out if possible (mine wasn't) or replacing.
A temporary get you going fix is to disconnect one of the wires from the pressure switch, but there is no safety cutoff if the pressure drops - replace asap.

Paul Newell, May 2006