|I'm assuming you are getting no video at all from the built in display, even while booting BEFORE Windows is supposed to load.|
If you DO always have video while booting BEFORE Windows is supposed to load but no video AFTER that, that's a software problem in Windows and is usually easy fixed.
Your problem can be caused by the backlight having burned out (probably most common), or by a faulty voltage inverter that supplies high voltage to the backlight (unlikely), or by the wiring between the video adapter and the display assembly having been damaged (probably second most common), or by the laptop having been otherwise damaged (probably third most common).
Have you, or is it possible someone else who had access to this computer...
- dropped it, or dropped anything on it, or otherwise exposed it to a physical jolt ?
- spilled liquid on it, or sprayed it with liquid, or exposed it to liquid such as rain ?
Since the video last worked properly all the time, has there been a power failure event that happened while the AC adapter was plugged in ?
Those things are the most frequent reasons something inside the laptop has been damaged.
Laptop no video (or video problems), and battery, AC adapter, power jack T shooting.
See response 1: