Your boiler is getting too hot too quickly because water isn't flowing through it properly, so it has safely shut itself down to prevent damage.

Flow temperature rise in heat exchanger too rapid. 1. Check the flow through the appliance and the system is filled. 2. Check for the air locks in the system and purge if required. 3. Check the isolation valves. 4. Check the pump is operating freely and check the voltage to the pump.