You should be able to control the build-in display from the F1 and F2 keys, located in the row above the 1 and 2 number keys on your keyboard. If you changed a setting involving the Function keys, you may need to hold down the "Fn" key (lower left corner) to make this work.
And also if it is an external display as is mine with a mac mini. It is normal with a mac for the keyboard controls to be the way to adjust screen brightness.
I think there was (is?) a bug in macOS Monterey caused by the screen resolution. If you switch to another resolution and then back to your preferred setting, the brightness slider should be back.