Need Help - Monitor too bright

Hi,

A couple of days ago, I was given a Dell E171FP monitor and it is so bright I can't use it for more than about 30 minutes at a time.  When I press the button at the front to control brightness and contrast, and then use the "-" key to tone it down, nothing happens.  I tried the "+" key to see if it worked and nothing happened.  Current setting for brightness is 90 and contrast is 70.  It's a used monitor and there is no manual.  I would appreciate your suggestions as to how I can resolve this problem.  Thanks.  Jane