If you can see details on the screen but it is dark, the problem is in the backlight system. Usual problem is in the backloght inverter.

Assuming you changed the inverter (it sounds like you did) check the input leads to the inverter - if you don't see 5-12 V, the mainboard is bad (DO NOT test the OUTPUT of the inverter - that is a much higher voltage).

If you DO see 5-12V going to the inverter, is the new screen the EXACT SAME model (same Dell part number) as you had?  If it isn't, is it the same TYPE (i.e., if you had an ambient light sensor, the new one needs one; if not, you can't use a screen with one) and a MODEL known to have shipped with this model system? 

If the inverter is getting power, and was changed - it's likely you've received two incompatible screens - they are not universally interchangeable.  The BIOS has a table of screen models built into it - no others will work.

Actually this just happened to me yesterday. I have an Insperon 640m and I had a problem with the touchpad, everything else was working fine. While trying to reach the touchpad I disconnected the display cable. I reassembled the laptop and tried to run it. I noticed the same symptoms you described. Screen is too dark and can only see shadows of windows starting up.  It works fine with an external monitor.

You need to take this to a repair shop to have it checked.

Can you please what the inverter is? and how does it look like and how can I distinguish the input leads from output ones?

Thanks.

Hi,

I have changed the inverter but still seeing the same faded screen. I think it is the backlight. Does anyone know anything about the backlight for the dell 640m laptop? I know how to buy it. It sells for like 12$. I just need to know where it is positioned on the screen and how to replace it.

Thanks.

How to replace a backlight bulb:

http://www.lcdparts.net/howto/troubleshooting/LaptopCCFL1.aspx

This is really helpful. Thanks.