The answer to 'boiler not turning on' is that there are several common causes, including a tripped circuit breaker, a malfunctioning thermostat, low water pressure, or a problem with the pilot light or ignition system. Symptoms may include the boiler not responding when the thermostat is turned up, no heat coming from the radiators, or strange noises coming from the unit.
If the boiler is not turning on, the first step is to check the circuit breaker and reset it if it has tripped. You should also check the thermostat to ensure it is set to 'heat' and the temperature is set higher than the current room temperature. If the boiler still does not turn on, it's best to call a professional HVAC technician to diagnose and repair the issue, as problems with the pilot light, ignition system, or water pressure can be complex and dangerous to troubleshoot without proper training and equipment.