Dell 6000 my laptop dell 6000 lcd dark i check lcd and invetor and cable but all ok i think this is motherboard parts problum but i dont no which part problum please help me .

If the display works fine when connected to external screen via VGA connector then then the problem is either with the LCD display panel ( which consists of the display panel and inverter ) or the LCD cable that connects the LCD to the motherboard. You could try to re-seat the cable connection on the back of the LCD panel.

power system on holding the letter D while pressing power button, (see if you have 4 solid colored screens displayed) if you do, then the LCD is ok, and this issue is likely a problem with the motherboard

Think you have answered your own problem here. If the external screen is ok, then the fault lies with the laptop screen.
Solutions..
1.) Remove keyboard top cover (download a "how to" guide if not familar with doing this)
2.) Remove screen bezel
3.) Check the inverter connections (inverter looks like a thin tube)
4.) Check cabling from motherboard to monitor /inverter for signs of fraying, tears, bare wires
5.) Check connections of the cable for damage.
If the above are ok, then you may have a damaged inverter. In any event, i'd look to repairing the damage to the loose lid, as its putting a strain on the wiring

try this press and hold Fn then tap on arrow up key check if the display will become bright. if fails try connecting an external monitor to your laptop check if display is fine. if not then uninstall video driver then install a latest one. if fails replace the video card drive or motherboard if video is built-in. if display is fine from the external monitor then reseat the LCD cable if fails then replace the LCD panel with the LCD cable.

You can reseat the LCD cable.

You can Cycle the Fn + F8 key combination

If no go, then replace the LCD

Run system restore to factory default.

Start the system and hold down the f11 key till you hear a beep. Then follow the onscreen instructions.

Good luck.

It's either:

the lcd
video (GPU)
motherboard
lcd cable.

It's really hard to say without being there in person. I'd recommend having a local tech check it out.

take this into geek squad to get monitor fixed because what has happened is that the back light behind the lcd has burnt out.

Connect an external monitor to the laptop and power it up, if you see a clear image on an external monitor, this means your video card is OK.

If you see a dim image or flicking image on the LCD screen it means you have a backlight problem.

Dim image and/or dark display on the laptop's LCD screen indicates a faulty LCD backlight and it could be the inverter that supplies high voltage to the CCFL lamp or it is the CCLF lamp is nearly burnt out or burnt out, most likely this is the case.
The inverter can be replaced easily but the CCLF lamp is more time consuming and requires soldering skills.
Check out www.lcdparts.net for parts, repair service and DIY info.

Check first by connecting it to an external monitor. If you see that it is the same problem in the external monitor then I think there is something wrong with your video card. See if you can re seat the video card.

This issues also come from the cable that connects the LCD with the motherboard. Have you opened the Laptop anytime and moved the cable?

Check these first. It sounds something loose or a something related to video card.