How do I unlock my monitor OSD lock?

I work in a school tech lab, and one of the computers has a screen that’s so dark you almost can’t see it (but it’s still functional). I suppose someone is pulling a prank-but when I try to adjust the monitor settings I get the message “OSD lock” and I can’t get it unlocked. I tried pressing every combination of monitor buttons I could, but I can’t unlock it.
Any ideas out there???