The answer to 'why is my light fixture' is that there are several common causes for a light fixture not working properly. This could be due to a burned-out bulb, a tripped circuit breaker, loose wiring connections, or a faulty light switch or dimmer. The symptoms you may notice include the light not turning on at all, flickering, or only working intermittently.
If the issue is a simple burned-out bulb, that's an easy DIY fix. You can replace the bulb yourself. However, if the problem is with the wiring, switch, or other electrical components, it's best to call a licensed electrician. Attempting electrical repairs without the proper knowledge and tools can be very dangerous and may even violate local codes.