Black screen with the white cursor flashing in the corner usually indicates a master boot sequence failure. Try testing your hard drive on someone's computer. If it's registered by the other computer then your problem lies with your motherboard or RAM. I would suggest a destructive format

My Pavilion is doing that right now, it was caused by trying the assorted device drivers for the display adapter... I reckoned I had Intel based chips and the drivers are causing odd results.... so far I don't have a good driver to increase the display properties, but restarting has made a difference, I've gotten several results thus far, so not dead yet thankfully. I did have the blinking cursor you mention...at that point, the PC required a reboot... hold the power button until shut down, most PC, regardless of age, allow this, rather then unplug anything.
Bill

If you're getting a master boot sequence failure, that means you have corupt or damaged windows systems files. And the fact that it was read my another computer as an external storage device only confirms it. Plug your hard drive back into a working computer and save any and all files you want to keep then do a destructive format on the harddrive