Thanks to their wonderful reliability, we all take light fixtures for granted and unthinkingly rely on their comforting glow whenever we need it, day or night. But what happens when a trusty fixture suddenly fails to light our way? Is it a bad bulb? Is it a wiring problem? Is it spite?
It's probably not that your fixture is trying to get back at you or receive negative attention (so you can rule out spite), and chances are it's something you can remedy without having to call an electrician. Check the following items in the order given here. (Tip: There's a better-than-90% chance you won't have to go past item 4.)
1. Plug or wall switch
Not to suggest that you're a dim bulb (pun intended), but have you made sure the fixture is plugged in and/or the switch is on?
2. Circuit breaker
Is anything else in the room failing to turn on? If so, check your service panel (breaker/fuse box) for a tripped breaker or blown fuse.
3. Light bulb
Before you replace the existing bulb, give it a twist and a jiggle -- sometimes that works. Otherwise, try a new one. And if the old one is a CFL (compact fluorescent), don't spend much time thinking, "It can't be the bulb; they're supposed to last 10 years." Some of them don't last 10 months.
4. Socket tab
If a new bulb doesn't work, turn off the switch or pull the plug (turn off the power, that is), and try bending up the little metal tab inside the bulb socket. These can get pressed in so far that they no longer touch the bulb's electrical contact. [Editor's note: Really, remember to turn off the power to the outlet before you stick your finger in it. We can't emphasize electrical safety enough.]
5. Lamp or fixture switch
Now that the easiest tests have failed, it's time to focus on the wiring. If the fixture is a lamp with an in-line switch (the kind with a wheel or toggle inside a little plastic case that clamps onto the cord), fiddle around with the switch and listen for any buzzing or other abnormal sound when you turn the wheel. These go bad all the time. If you sense anything amiss, replace the switch with a matching part (just follow the manufacturer's directions). You might have to replace the cord, too, if the copper wiring is damaged.
Pull-chain switches on mounted fixtures tend to fail before other parts. If the switch has felt funny recently or failed to turn the light on or off occasionally, it's probably a bad switch. Turn off the power to the fixture's circuit at the service panel, remove the fixture, and replace the switch with a matching part (provided you're familiar with basic wiring).
Both plug-in lamp cords and cords that tether hanging fixtures can come loose from the socket terminals, the internal fixture wiring, or the plug terminals. Move the cord and/or fixture into a range of contortions (get creative!) to see if the light goes on even for a moment. If so, check the cord connections. If those look ok, replace the cord.
You can test a socket for power with a $3 circuit tester: With the power to the fixture on, touch one tester probe to the metal socket tab and the other to the threaded metal sleeve that the bulb screws into. If the tester lights up, the socket is ok; if not, it could be a bad socket, a loose (or fried) wire en route to the socket, or a bad wall switch. If items 8 and 9 pass the test, replace the socket.
8. Wall switch and fixture cable
Before taking the fixture apart, you might as well check the wall switch and wire connections between the switch and fixture cable. You can do this with a circuit tester, but only if you're familiar with switches and basic household wiring.
9. Fixture wiring
If the switch is getting power and is sending it to the fixture, it's time to take apart the fixture, as needed, to check the socket terminals and other wiring connections. Chances are it's just a bad socket or one of the fixture wires has fried due to "someone" ignoring the wattage rating on the fixture and putting in overly bright bulbs time and again (that, Mr. Edison, is why there's a wattage rating in the first place).
10. Circuit cable
If item 8 proves that the switch isn't getting power (and you're sure it's not due to a tripped or faulty breaker or blown fuse), you might have an interruption somewhere along the circuit path. It's probably time to call an electrician. At least you can't blame it on your reliable old fixture.