Your boiler has shut down because there is a significant difference between the water temperature leaving the boiler and the temperature returning to it, usually caused by a circulation problem.

Water shortage, temperature difference between flow and return NTC too large. Pump blocked or defective, pump output too low, flow and return NTC interchanged

What causes this fault?

This fault occurs when water is leaving the boiler much hotter than when it returns, which suggests the water isn't moving through your radiators quickly enough. The most common reasons are a failing internal pump that can't push the water effectively or a blockage caused by a buildup of sludge and debris in the pipework.