Your boiler has shut down because water is not flowing through it properly, which usually means the system needs topping up with water or a valve is closed.

Flow rate in heat exchanger too low 1. Supply sensor is not assembled on the supply pipe. 2. Supply sensor does not make proper contact to the supply pipe. 3. System is not filled with water. 4. No water flow, because CH system is closed. 5. No water flow, because pump is not operating correctly.