I have a 1999 Honda Civic. The low beam headlights (both) stopped working simultaneously. The high beams work; the parking lights work; all other lights work. I find it unlikely that both would burn out at the same time or that Honda would be dumb enough to wire them in series. It appears as though the left and right headlights have separate fuses, so they can’t be bad. Anybody have any ideas?
This happened on my '75 Civic, back in the day. Turns out both bulbs were bad. They did not burn out while on. I think the filaments broke when I hit a bump somewhere. Just turned them on one evening and no low-beams.
Have you checked for voltage at the fuses?
The following applies to non-DSL (Daylight headlights):
Either, try a replacement light bulb in each position, or use a test light at the light sockets. If the fog lights come on, the light switch is providing power to the low beams and the fog lights. If the fog lights don’t light, AND the low beams don’t work, the light switch is likely bad. It’s not impossible for both fuses for the low beams (or bulbs) to burn out, the fuse for the fog lights, also.
It is likely that one burned out some time ago, but you did not notice it until now. Start off my changing them, the typical 10 year old car is due for new headlights anyway.
I came across this post searching for the answer to the same question. I have a Geo Prizm 1990. Low beam worked just fine last night (or, at least one of them, but I think both worked because we had a bad blizzard weather and I did not see anything unusual about how the lights worked). But they both would not go on immediately after I started the engine in the garage this morning. On my car, there are separate fuses for the left one and for the right one, as well as a relay that says “Head”, all under the hood. I have checked the fuses and they are OK. I can hear the relay click when I turn the switch. I don’t know what kind of sound the relay is supposed to make but it sounds the same when I try high beams and they do work fine. However, the indicator (blue light on the instrument panel) does not turn on even though/when the high beam is on. My first reaction was also that it would be highly unlikely for both low beams to go bad at the same time. In my case, you twist the switch on the steering column for low beam and push it for the high beam (away for continuous or toward yourself for as long as you hold it so). I want to ask you to make sure: is it possible for the switch to be bad for the low beam and work OK for the high beam? 
I can only hope that it is just the bulbs because I cannot afford any other repair (it’s like “total loss” for a car this old).
When the high beam works, how about a small indicator light on the instrument panel? (see also my reply to hellokit’s reply in this thread)
Also, when was last time that you changed the head bulbs? For me, it has been a couple of years, three years max.
Bought and changed the bulbs. That fixed it. Thanks to everybody on this thread.
A learned physician once told me the following little tale:
“In US medical schools, one of the first things that they teach us is–if you hear hoof beats behind you, it is wise to first suspect horses before you jump to the conclusion that the noise is from zebras.”
In other words, always try to eliminate the most obvious causes first, before jumping to conclusions that the problem results from a more obscure cause. In this case, the most obvious cause turned out to be exactly what the problem really was. As Mr. Meehan sagely stated, you probably did not notice that the first headlight was burned out until the second one quit.