Monitor Brightness

[Borderlord]

Hi Guys
Trying to adjust the brightness on the monitor.
Tried through settings, got to brightness slider, but that won't move.
Tried again through Action Centre, got to the same page, but still slider won't move.
any ideas?
Jeff

[camerongray]

Is this a laptop/all-in-one with a built in display or is it an external monitor?  That slider only works for monitors built into the computer, with external monitors you can adjust the brightness using buttons on the monitor itself.