Dimmed screen that is locked by password
The screen dim controls on the side of the device won't dim the display that much. It might be a backlight failure and the device could need a repair.

You can take the device into a room with a table lamp and usually shine the light on the side to see enough to operate a screen even with the backlight totally failed.

You can unlock the device by connecting it to the computer that it normally syncs to in some cases as well.