If you can display ok on an external monitor then your video card must be ok. It appears you may have a problem with your LCD. Sometimes, there is a small plastic piece near the hinge where if it is depressed it indicates to the notebook to shut down the LCD.
Check out this possibility.
I have a HP PAVILION N5250 PIII 700 MHZ 128 MB RAM 10 GB HD CD/DVD PLAYER AND AC ADAPTER, I just bought it real cheap because a LCD TROUBLE and is the following.. the laptop powers up good, boots up and runs great but the LCD stay BLANK, no video at all but the laptop is working because i can hook up the laptop to an external 15" monitor. any idea what's the issue with the video or LCD..?

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