Sounds like you have a Bad Display or a Bad Display Cable/Ribbon/Connection. It's not the Video Card since you can view on a a External Monitor. The Display Cable/Ribbon is the Cheaper of the two so I'd try and Replace that first(or at the very least check and make sure the cable/ribbon connection is seated properly on both sides display side/motherboard side). If you have the ribbon check to see if there's crack on it. if there is...good possibility it's the problem
If you look into the screen and see no backlight but can see text/graphics then we know it's still getting data and that's when we know it's not a data cable and possibly the inverter board/light but if all you can see is the light and no image whatsoever. It's either the display/data cable/ribbon or the display itself.
If you can't view through a external display, then it's the video card which is typically integrated onto motherboard.
1. Seat cable/ribbon properly and check if a crack on the ribbon(no money needed for this step).
2. Replace Cable/Ribbon(inexpensive compared to replacing screen).
3. Replace Display
Yes I did and in my case it was malfunctioned onboard nVidia graphics chip because no external means worked for trying to output signal. I could have chip extracted from motherboard and replaced with new one which I could order from eBay, I could pay DELL authorized service center over $800 to have motherboard replaced due to expired warranty but only paid them $60 for basic checkup which led to quote of over $800 from them. I could easily buy same new model from eBay so what I did was ordered motherboard on eBay for $130 and personally replaced old with new motherboard thus saving $670 and enjoying the experience along the way.
Sounds like my problem might have been a little different. Here's some details on my symptoms and how I got it working again.
Hope this helps someone else down the road!
For reference, here's the E6420 service manual which aides in disassembly: