Okay...
1. What color is the screen actually
2. Does it have anything on the screen, typed letters or anything of the sort
3. Did it just happen randomly? While performing a task? Or after doing something, like downloading a file.
4. How long was PC running before this happened?
5. Does it do it all of the time?
6. When it does it, does the screen stay there until you shut it off, or does it flash and then shut down itself and reboot automatically.
That will help us.