The answer to 'water heater not heating' is that there are several common causes, including a faulty thermostat, a broken heating element, or a problem with the water heater's power supply. Symptoms may include lukewarm or cold water, strange noises coming from the tank, or the water heater not turning on at all.
Troubleshooting a water heater issue often requires some basic DIY skills, but in many cases it's best to call a professional HVAC technician, especially if you're not comfortable working with electrical or plumbing systems. A qualified technician can diagnose the problem and make any necessary repairs or replacements to get your water heater back up and running.