Common reasons why a boiler does not light include an electrical issue, interruption of gas supply, an incorrectly set thermostat on the boiler or a faulty boiler component. For the boiler to work, the thermostat should be set to the heat position and lower than room temperature to activate.
Electrical issues that may plague a boiler include a tripped breaker or a safety switch that has accidentally been tripped to the off position. Electrical components that may have malfunctioned include the electrical ignition (if the boiler is equipped with one), fan blowers and circulation pumps. Interruption of the gas supply from the shut-off valve being placed in the incorrect position, impeding gas flow, can cause the boiler to not work. A faulty thermocouple, sensor, control valve or other component degraded by normal wear and tear may cause the boiler to not ignite.