Computer problem when I turn it on a black screen appears with a white minus sign on top left corner of screen?

That means the Hard Drive is not working Your system cannot find a bootable device Save important files and reload Windows may help for a while Your hard drive may be failing

That minus sign is most likely an underscore ( _ ) and what it probably means is that you had a virus that ate your operating system!!! It is entirely possible that this is not the case, but this did actually happen to me once. Another reason might be because it's having trouble booting up. I am almost positive that every computer actually does this, but usually they are too fast. While I'm not a computer expert, I would recommend cleaning the system and re-installing your hardware after you secure everything that is important to another location.

It's not a minus sign, it's a command line. It's waiting for you to type something in. If it stays at that screen, that means it's having trouble loading up Windows Maybe your computer gets very hot sometimes? That's what mine use to do, and it caused my computer to do that

If this only happens sometimes it sounds like your POST test isn't starting up or it is but it's not loading the OS. Get into your BIOS and run a disk check to see if anything is wrong with the PC.