China manufacturer of magnetic ballasts for metal halide flood lights & hps flood lights

A failed fluorescent light fixture can be caused by lack of electrical power (tripped breaker or blown fuse), a dead or dying fluorescent ballast, a dead starter or a dead bulb(s). Check for power first... then the starter (if applicable) and then the bulbs. When all else fails, the ballast should be replaced. Since it is the most expensive item, be sure it really is dead!! Ahd check the price before you buy... some ballasts are more expensive than new fixtures.

When flickering is the issue, you still must do the same sort of troubleshooting since all the same problems that can cause a lamp to not work can also cause flickering... defective starters, defective bulbs or a defective ballast.
If the fixture contains a single ballast, try replacing the ballast.
If the fixture has multiple ballast's and none of the lamps work, the problem is almost certainly a lack of electricity or low supply voltage. Use an AC voltmeter to verify that the right electrical voltage is reaching the ballast. Do not measure at the lamp sockets or starter socket. The correct voltage for the ballast should be marked on the ballast.