i accidently let my laptop on too long and it restarted itself to a black screen with an underscore on the top left corner and wont let me do anything.
well, depends on what's damaged.
this seems like BIOS related but lets just say it is not.
download Hirens Boot CD: http://funadvice.com/r/bukasepi45s
its a collection of freeware and trial ware applications with a built in Mini windows XP (which boots from the CD and not your Hard disk)
burn this on a blank CD (the .zip archive has a quick burner application on there so its very simple)
now stick this in the laptop and boot it up (press F12 or whatever to load the BIOS configuration and set the Boot device to your CD/DVD drive, restart your PC)
once you are in mini XP use the command line (write CMD in the Run box) to write : /chkdsk /f
This should theoretically fix any issues you have with booting up,
unless you fried a circuit because the CPU vent was blocked or something (oh right it could also be the CPU O_O)
oh right, and if you dont know how to boot from CD or use the Command line you can find a LOT of helpful tutorials online