Random Vista Failure

I was watching a movie from an external hard drive connected to my PC running windows vista and it just shut off.

When starting it back up again I could hear all the typical sounds of fans/HDD spinning during the boot process yet after the BIOS screen passed I am met with a black screen with a single blinking "_" at the top. Nothing can be done from this screen, no commands, function keys, or anything else is possible.

I disconnected my external drive, rebooted and entered the BIOS diagnoses and which signaled that CPU, RAM, and HDD where all functioning as they should.

Inserted vista OS disk and tried to repair startup but it says it cannot fix it automatically. I tried to simply re-install windows after all this and find that vista boot disc is not recognizing my SATA WD drive.

I only have basic trouble shooting skills and this seems beyond my knowledge, if anyone has any suggestions to offer they would be greatly appreciated.

Thanks.