What to do if your head light fail on your car?

Take out the bulb and inspect the filament. If the filament is broken, replace the bulb. If that doesn't do it, then check the fuses and relays - your owner's manual should tell you where to find these. You can either visually inspect your fuses or use a test light (for a faster way of doing it). For the relays, you'd need to take a digital multimeter and test for resistance. If you don't know how to do this or don't have the means to, then you can have it done at any major auto parts store (NAPA, Advanced Auto Parts, O'Reilly, Auto Zone, etc.). If that doesn't solve your problem, check the connectors on the wiring - it's possible one of them may have gone bad. If that doesn't solve your problem, check your headlight switch - find out which poles on the back of the switch create the closed circuit to turn on the headlights (the wiring diagram in a Chilton or Haynes manual can help you determine this, or a factory repair manual if you can find one at a reasonable price), turn the headlight switch to the on position, and test the circuit for resistance (using a digital multimeter). If that doesn't solve your problem, then you're going to have to trace the wiring and find out where the defect is.