Thermostat display is completely blank and unresponsive; heating does not operate.

Safety first: This issue involves mains or 24V power. Turn off the relevant breaker before removing the thermostat or touching any wiring. Use a qualified electrician if unsure.

Steps to diagnose and fix:

• 1. Check breakers and fuses: Inspect the consumer unit and any local fused spurs feeding the heating controls. Reset tripped breakers and replace blown fuses with the correct rating.
• 2. Verify backplate power: Remove the thermostat head from its Danfoss backplate. Using a multimeter (only if competent), measure voltage at the power terminals. Compare to the required supply (e.g., 230V AC or 24V AC).
• 3. Tighten terminals: With power OFF, ensure all live, neutral, and low-voltage conductors are firmly clamped and that no wires have pulled out of the backplate.
• 4. Check transformer (24V systems): Locate the 24V transformer feeding the thermostat. Measure its output and replace it if no voltage is present.
• 5. Replace thermostat head: If correct voltage is present at the backplate but the display remains blank, the thermostat's internal power supply or display module has failed. Replace the thermostat head unit with a compatible Danfoss smart thermostat.

When to call a professional: Any uncertainty with live measurements or wiring requires a qualified electrician or heating engineer.