Laptop display is not working.

Here are some quick points that I'm hoping will provide clues to help solve my problem:

15R.  N5110.  Has been my trusted companion for about 3 years.  Love my Dell.  But...

My laptop is almost always on.  I left it off for 3 days and the disply does not work now. (oddly enough, perhaps coincidence, the same exact thing happened to me last time i left it off for several days, not sure if that's related).  last time, i bought a new display and it worked.  i really don't want to do that again. 

When I start it up, the display comes on for a split second to reveal a box that I don't usually see but it flashes off before I can read it.  Display then goes very dim.  NOT black but very dim, like a charcoal grey, you can tell it's on but nothing ever happens.   I hit D when powering up and the screen does turn several solid colors one by one.  Don't know what exactly that means but I read to try it.

I have tried to hook up to an external monitor - it doesn't display my computer screen.  Possible that it is my cable but not 100% sure.    I can test with another monitor if it will help my cause.

I would greatly appreciate any assistance that can be provided.  Thank you in advance.  I would love to have my computer back.