The cable degrades over time and use. Replace the cable from mainboard to LCD panel.
About an hour job. Cost is about 200 at a good shop. Much less if you do it yourself. Might be a beginner task since it's all just screws and connectors.
Tip: The cable is likely to be a flat cable. To unplug look carefully for a little slide up release. It's sometimes quite small so I use a small flat blade to push it open. Then the old comes out, the new goes in and the retainer is pushed back in. Also, never touch the contact surfaces with your hands.
I have a Compaq Presario 900 bought in 2002. The monitor works when the lid is tilted an inch or two above the keyboard. But when it is raised further the monitor is very dim. What could be the problem? Worn connection, backlight or inverter? How would you diagnose it? Could the repair be done by a beginner?