Your boiler has switched itself off because it has detected dangerously high temperatures, likely caused by a disruption in the flow of water circulating through your radiators.

Technical description

Overheat thermostat lock-out. Relating to thermostats, thermal fuse, or manual reset thermostat. Check for water circulation. Check for air in the heat exchanger. Check for system restrictions and blockages. Check overheat thermostat and connections. Reset overheat thermostat if required