Your boiler has shut down because it is overheating or detecting that water isn't flowing through the system properly, so it is taking a short break to prevent damage.

Technical description

Lock-out: appears if the maximum temperature rise is exceeded or if the dT between the supply and return temperature ≥ 45°C or a rate of rise in the flow temperature > 1°C/sec. And also at a boiler-start with no through flow or insufficient water. The boiler will try to restart after 10 minutes.