If your fixture smells of burning electronics, or if smoke came out of it when it failed, or if the ballast has scorch marks on it or tar like stuff dripping from it or oozing out of it the ballast in your fixture has failed, See ballast replacement. Warning: the smoke or tar from a ballast that has failed and contains PCBs is highly toxic. See ballast replacement for complete warning information.
If you have just replaced the ballast in your fixture, make sure that you have connected the right wire lead to the right bulb. The ballast label has a wiring diagram that you can use to make sure this is right. If you have reversed the wires and if you have left the fixture turned on for extended periods of time you may have burned out 1 or more of your lamps. Make sure that your ballast is properly grounded, that is, the ground wire is attached. For more information, see the ballast replacement section. Also make sure that the lamps are fully snapped in and that they are within a half inch of the metal part of the fixture. You can bend the clips to achieve this. Before performing the following final check, shut off the power at the circuit breaker box. The final check is that the black wire from the ballast is connected to the black wire from the source wiring and the white wire from the Ballast is connected up to the white wire from the source wiring. (note: in a lamp the wires may not be color coded. Try both possibilities. If none of this helps, contact the manufacturer of the ballast or get professional help.
If none of the above information has helped, you probably need a new ballast. See the ballast replacement section.