As it's OK with an external monitor, the problem is most likely the backlight. If you can see a dim screen display when shining a light on the screen, the backlight power supply (inverter) needs to be replaced.
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Thanks for the response.
Unfortunately, do not see a dim display, even if a flashlight is directed to the screen.
If I make the room completely dark, the backlight itself is visible, even though nothing appears on the screen. This is similar to if the computer is on standby.
Think of it like having a TV on, but with the cable box turned off.