Which Windows service controls screen brightness?

I have this problem too. A reboot fixes it, but I'd rather not have to shut everything down. (If a reboot does not fix it, you should reinstall your drivers.)

I tried restarting the DisplayEnhancementService as suggested in another answer. It didn't help. I also tried restarting the graphics driver with Win+Ctrl+Shift+B. This didn't work either, but it's a good thing to try when your screen isn't turning on.

However, I was able to fix this issue on my laptop without a reboot by disabling my monitor:

Start Device Manager (I use Win+X M). Select "Show devices by type" in the View menu, and then find your device under "Monitors". On a laptop with no external displays attached, there should be only one. With the device selected, click the "disable device" button in the menu bar, and confirm in the popup dialog. This did not turn my display off when I tried it, but it flickered. Then enable the device again with the button. My brightness controls worked again at this point.