Find the manuals for your car here:
http://www.nicoclub.com/archives/nissan ... nuals.html
and check out section LT for information on the lighting system. There are four fuses (separate high and low beams fuses for each headlight) and three relays (two separate high beam relays and one for both low beams) involved. There may be a problem with the headlight switch, any of the wiring involved, or of course a filament in either bulb could be burned out. Section LT should help guide you towards narrowing down where your problem might be, and a mutimeter is good to test things with.