Several reasons: Bulb may be burned out. Remedy install a good bulb.
Switch may be in the off position. The switch is a 3 position switch. Off, means that it will not light, Door which means that when a door opens the light will come on, and on, the light stays on until it burns out or loses power.
Everything in the car should be protected by a fuse of some sort. Check your manual to see if the fuse is out.
Finally, the door switch may be broken. This can be remedied by using a can of WD40 and working the door open/close mechanism several times. Use a screw driver to work the mechanism so you do not have to open and close the door.