The answer to 'smoke detector not working' is that there are several common causes, including a dead or disconnected battery, a malfunctioning sensor, or a problem with the electrical wiring. Symptoms can include the detector not beeping when tested, no LED light, or the detector not responding to smoke.
Some simple troubleshooting steps you can try at home include replacing the battery, vacuuming and cleaning the detector, and testing it with a smoke detector tester. However, if the problem persists or you're not comfortable working with electrical components, it's best to call a professional electrician to diagnose and repair the issue.