I accidentally disabled the screen

You dont say which OS you have so here are links for password removal for
XP

Vista

Windows 7

Severe corruption. Since you can no longer boot in any way in it it's going to be extremely hard to fix it. Better just reinstall Windows from scratch. Use the recovery cd that should have come with the laptop. If you don't have it then just use a disk borrowed from a friend and the key on the label.

You need to download from DELL website, since there are more than one versions of the 8200, you should use the service tag instead, see here:
http://support.dell.com/support/downloads/index.aspx?c=us&cs=19&l=en&s=dhs&~ck=mn

Immediately after seeing the Dell logo, keep tapping the 'F8' key,
until you see a "menu" (white text over a black background)
giving "startup" options for Windows XP.

Select "Safe Mode" and see if you can start in that mode.
Then, use "System Restore" to go "back" in time, to the most-recent checkpoint (date/time) where your computer _DID_ work.

Restore to that point, and restart.

it sounds like your display backlight has gone bad. If your handy you can get a new display. If you are really good you can replace the back light.

mybe you have accidentally press the dual video mode...in your unit there is a FN kay located near the ctrl and alt button....and from f1 to f12 locate that button that has a logo for minitor by pressing at the same time the FN key and that screen logo you find press it and release if see if that works.

sounds like a bad driver-can you boot into safe mode? how about the bios?

plz try to boot through safe mode, if you can then log in normally.
if you cant boot from safe mode then you have to reload or repair your os.

I have the same problem with my Inspiron 8200 - it's the video driver. The system will load but then no log on screen. The solution until you can get an updated driver is to do a power-off reset.

Press and hold the Fn key, then press F8. See if this brings up a list of options and select the "Display only" option.
Ron-