Most Windows 8 computers have a temporary black screen, maybe a POST type screen, before the login screen pops up. Can you then login and everything works fine? Or are you saying the login screen never appears? Unfortunately, you haven't told us much about the computer.. Brand name? Laptop or desktop? Windows 8 or 8.1? Why did you format the computer? What kind of problems were occurring?
Also, although you didn't say it, I'll guess you didn't just "format the pc" but also installed the operating system, etc.? Was the a reinstall using a recovery partition or did you reinstall the OS using a Windows 8 disk? Installed all the drivers too?
Hope this helps and let us know more.
Formatted my computer today and it's still black screen before the login screen, it was like this before I formatted my computer.. what is wrong?