The usual press time+ and select wasn't working, the battery inside would prevent a full reset so this procedure does a total system reset.
Mains off Unscrew with flat screwdriver the holding screw at the bottom of the unit lift unit off backing plate pulling the bottom towards you using a thin knife or a screwdriver lift off the back panel to expose the circuit and the round backup battery with a small bit of metal ie your screwdriver breifly touch the contact below the battery to the outer case of the battery ( temporary short ) replace the cover, the unit is now reset to default ( you should be able to see the display working again if you hold it at a angle ) place back on wall tighten screw turn on mains