Screen Turns black on login.

When I turn on my computer and goto the login screen it turns black. I hit the Fn+F5 to get it so I can login, but after I do that it goes onto my account then turns straight into power saving mode. I do the Fn+F5 there, but if it works then three seconds later it goes back into power saver and doesn't turn back on at all. It only does this on my sign in and my mothers the other one we don't have the pass to do to us buying it from a relitive that has passed away. Please help us fix this problem. Ty.