First of all, make sure you are not using your battery and are plugged in. You will need to take an externalmonitor and plug it in to your laptop. Boot up your laptop and see if you can see the desktop on the external monitor. Note from the original post you may have to toggle [Fn F8]to see the external monitor. If you can see your desktop, then the question is, itthe backlight or the inverter. Be sure to look at the first post and make sure all your drivers are up to date, etc.
When I checked with a local computer repair tech, he told me they change the inverter first and if that does not fix it, they send it out for the backlight. I was able to download the manual on my laptop [from Dell support] which showed detailed instructions to dismantle and replace my inverter.[Be prepared to take out lots of small, eyeglasses size screws] A replacement inverter was about $45 on ebay. The backlight, however, is supposed to be replaced in a clean room. It's not a cheap fix.