More than likely there’s a function key or a function lock which is on or stuck. Check in the lower left hand part of the keyboard for an fn key. Press and hold that and see if you can get the brightness to work. If yes, then function lock is probably enabled (some models have a light on this FN key to indicate this, but I don’t recall if this is one of them).
If there is a light, or it seems to be enabled. You can turn function lock off by pressing and holding the FN key and then pressing shift (which should say FN lock in the corner in addition to Shift).