The answer to 'water heater not working' is that there are several common causes, including a faulty thermostat, a broken heating element, sediment buildup, or a leak in the tank. Symptoms may include no hot water, lukewarm water, or water leaking from the tank.
If the issue is a simple thermostat or element problem, you may be able to DIY the repair. However, if the tank is leaking or the problem is more complex, it's best to call a professional plumber to diagnose and fix the issue to avoid further damage or safety concerns.