Two things I can think of. Could be your graphics card or bad memory. Could you please provide some specs on your system.

1.How long does the screen stay black ?
2.Are there any external USB devices plugged in
3.When was the last time you made system updates
+ What Uvah said kindly post system specs

There are several possible causes for something like this. Can you remember any details about how the problem started, and when (for instance, after it received a blow, etc.)

You should post the brand and model, when it was purchased (originally) all your system specs including the OS, and when you updated your drivers (and from where they were updated).

if the comp is a laptop then windows might be set to use the external screen as the default screen while the bios/boot is using the internal, but windows also does like to switch graphic modes from the loading screen to desktop/login res, but not update the image for a short time(depending upon the system spec might be upto a couple of minutes(more ram can reduce this time in my experience)),

if the comp is a desktop/tower it might have a motherboard video AND a seperate video card with windows useing the video that is not connected, but bios/boot/safe mode all using the connected video,

BUT to better help you we NEED more info about the computer

harpo