Your boiler has lost communication with its external controls like your thermostat because there is an electrical problem with the wiring connecting them.

eBUS low voltage fault (short circuit)

What causes this fault?

This fault occurs when the low-voltage communication line between your boiler and thermostat is interrupted, usually by a short circuit. The most common cause is a loose wire at a connection point or moisture getting into the external control box. In some cases, it can be triggered by a fault within the boiler's internal computer board failing to process the signal.