The answer to 'why is my boiler not turning on' is that there could be several common causes, such as a thermostat issue, a problem with the pilot light, or a malfunctioning component. Some common symptoms include the boiler not starting at all, the pilot light going out, or the boiler making unusual noises.
If your boiler is not turning on, the first step is to check the thermostat. Make sure it is set to the correct temperature and that the batteries are not dead. If the thermostat seems to be working, you may need to relight the pilot light. This is a relatively simple DIY task, but if you're not comfortable doing it, it's best to call a professional HVAC technician. Other potential issues, like a malfunctioning pressure valve or a problem with the heating element, will likely require the expertise of a professional.