A boiler may not work because it doesn't have power or the water level may be too low. Other problems that may cause a boiler not to work are a malfunction of the pilot or electronic burner or a malfunction in the thermostat.
If the boiler fails to work because it doesn't have power, it may be because the breaker is tripped or because the fuse that controls the furnace is blown. To fix these issues, reset the tripped circuit breaker or replace the blown fuse. A boiler that isn't working because the water is too low can be fixed by maintaining the water level of the boiler at half full. The boiler should have an automatic filling system that uses a pressure release valve to maintain proper water levels. If the boiler doesn't have a pressure reducing valve, it can be fed manually by opening the water feed valve until the boiler's pressure reaches 12 pounds per square inch.
If the boiler isn't working because the pilot light is out, simply relight the standing pilot. Troubleshoot the electronic ignition if the boiler doesn't have a standing pilot. To fix a boiler that doesn't work because of thermostat issues, first check that the thermostat is in heat mode. Make sure that the thermostat is set to an appropriate temperature setting, and try moving the temperature setting up or down a few degrees.