HEADLIGHT PROBLEMS

Both headlights are not working-- 
This is usually caused by lack of voltage supply to the headlights due to

• wiring fault
• bad headlight relay/fuse/headlight switch/dimmer switch/module

To know the exact cause of the headlight problem, retrieve your owners manual and locate the main fuse for the headlight circuit. If it'd blown, replace fuse with a new one but if it blows again like almost immediately, then there is problem in the headlight circuit that will have to be diagnosed and repaired before headlights will function again. But if the fuse does not blow and your headlights are still nor functioning, this is likely because there is no power at the fuse which would indicate a fault in the wiring either in the fuse block or between the fuse block and the battery
If there is voltage in the fuse, then it's time to inspect the headlight relay. You do this by pulling the relay out and shaking it. If something rattles inside the relay it means it's faulty, time to get another relay

One headlight does not work--This usually means the headlight bulb has burned out. Before replacing the bulb check for signs of corrosion or damage on the electrical connector on the back of the headlight
Most modern vehicle have headlight that changed from the back of the headlight housing. It is easy to access this housing on some vehicles while on some others you may have to remove lots of components before you get to the bulb. See your owner's manual to better understand your vehicle's headlight housing
NOTE: When changing the bulb, make sure it's the same size and style as the original one
CAUTION: Never touch the glass on a new halogen bulb with your finger because the oil in your finger can react with the quartz glass and cause the bulb to burn out prematurely, ALWAYS USE GLOVES