Your boiler is taking a short rest to prevent it from switching on and off too frequently, which protects the internal parts and saves energy.

DIY fix possible

Check that your room thermostat is turned up high enough to call for heat.Ensure at least one or two radiators have their valves (TRVs) turned to maximum to allow heat to circulate.Wait 15-20 minutes for the countdown to end, as the boiler will restart automatically.