Q: I changed the lightbulbs on my cars headlights but the driver side headlight won't turn on...

Hello. Ironically, I happen to have a 2000 CLK 430 as well and I experienced an almost identical scenario as you did. I had a fog light out on my vehicle, thinking it was a bulb I went ahead and replaced the light bulb but once I replaced it it still did not fix the issue. Upon further inspection of my vehicle I found that the some of the insulation around the wiring leading to the fog light was brittle and causing wires to be exposed. I taped the wires with some electrical tape because Mercedes-Benz has a very sensitive electrical system and if a wire is exposed it will cause resistance. If the electrical system senses too much resistance it will cut power off to that component to help protect other electrical parts. I suggest to have a technician such as one from YourMechanic inspect the wiring of your fog light that is out, check the fog light fuse in the side fuse compartment, and also check the clearance light bulb. The clearance light bulb is a small bulb located under the fog light and is integrated into the fog light circuit. If the clearance light is out it will interrupt the fog lights power. On my vehicle it was a clearance light being out and the damaged wiring to the fog light causing my fog light to be inoperative. You are able to find a technician and schedule an appointment to evaluate your vehicle here https://www.yourmechanic.com/services/fog-or-driving-lights-are-not-working-inspection