Headlights (low beam)

There may be a 15a HDM fuse, #53 on your 2004 GMC Envoy. My service information shows it on "late production" vehicles. It would be in the same fuse box as the 10a headlamp fuses. If that fuse is good, or not used on your vehicle, the nest step would be to check for power at the headlight fuses using a "test light".

Did you try changing one of the low beam bulbs http://partimages.genpt.com/largeimages/1017965.jpg as you could have been driving with one bulb out and didn't notice until the second bulb went out. The high beam and low beam use different bulbs