Could be a corrupted account. See if you can boot into Safe Mode. Then make a new administrative account, login into that, the logoff and reboot and see if you can still login into that new account.
Product Name: HP Pavilion 15 Notebook PC
Operating System: Microsoft Windows 8.1 (64-bit)
I press the power button.
Computer powers up.
I see the Windows 8.1 logo and the doting thingy that goes in a circle.
After a minute, when the password screen is supposed to show up it simply goes blank (black).
Please help and thank you.