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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 ?
Has there been a power failure event that happened recently while the AC adapter was plugged in ?
Those things are the most common reasons something has been damaged inside your laptop and must be replaced.
Laptop no video (or video problems), and battery, AC adapter, power jack T shooting.
See response 1:
DO NOT plug in or unplug a monitor's video cable into / from a video port while the computer is running. Doing so can damage the monitor's circuits and/or the video adapter's circuits.
Some laptops will display on an external monitor without you having to do anything.
If you get no video on an external monitor, some laptops have a key combo that you must press to toggle the display to external video only / built in display video only / sometimes both - see the Owner's or User's manual for the model.
If that doesn't help, your video adapter is damaged (fried), and/or your mboard is damaged (fried).
You should get video while booting BEFORE the operating system loads in any case on the external monitor.
The video adapter is built into the mboard except on high end (much more expensive) laptops - in most cases you must replace the mboard.