ubuntu boot problem

ok, one day i booted ubuntu and every thing went fine but i had to turn my computer off with out useing the shutdown thing, but after that when i try to boot in to ubuntu i just get a black screen! i looked on the web and found nothing that matched my problem so is there any way i can fix this?details:
im booting of of a 320 gig usb external hard drive (my book, WD)
i have ubuntu 8.04 LTS installed

do you still have the live CD? if so then try booting off that and reinstalling linux, or it may have a repair option. Im running Intrepid 8.10 and ive never had a problem with forced shutdowns.

it dident tern off by it self i turned it off my self with out going through shut down

If you have Windows running on said machine, start up Windows and reboot back into Ubuntu. It worked whenever I had the same problem. Otherwise search on the ubuntu forums.

how do i do that