Screen is pitch black and will not work.

I just got my sisters old iPhone 6 last night and it was dead so I had it on the charger all night. However when Tried to turn it on this morning the screen was black. The home button vibrates whenever I press it but I don’t see anything. I can also hear typing when I touch the screen as if I were putting the password in but the screen is still black. I believe the brightness may be all the way down and that may be what’s wrong with it but if it is, is there any way I could get into it and fix that without iTunes or anything, because it has not been used in idk how long so I don’t know any of her passwords or anything. Would I have to take it to the apple store or something or just get a whole new phone?