are you running higher wattage bulbs in it? if so, take them out and throw them away. they burn your wiring harnesses. not immediately- usually takes about 5-6 months- but eventually you cook the harness and end up having to replace it.
usually the damage is the socket itself that plugs into the light. stick with the standard wattage bulbs if you don't want to go through the hassle. look for a burned pin on the headlight socket..
If you're still on standard bulbs, also check the light switch inside the car. it has separate contacts for each side on high and low, plus parking (so five or six contacts total). often they can get dirty and stop working, and you'll have one light go out as a result.
check both of those things and see what you come up with.