I saw a similar topic while searching the forums that i assumed was completed.
I was running Firefox and Don't Starve (Steam) when I got a popup saying my Java version was insecure. I then got another popup from Windows saying something to the effect of it had detected a security problem, at which point it quickly shut all programs and then itself down. After a pause, it restarted itself and brought up a blank screen with a flashing underscore in the upper left hand side. Thinking this was a command prompt, I tried typing, but typing does nothing. In fact, the caps lock and num lock on my keyboard don't even work. Pressing the power button in this state immediately turns off the computer.
In the similar topic (the guy was also running Linux but only having this problem with Windows 7), the solution was to find your hard drive in the boot options of your BIOS and boot from that. I tried booting from every boot option in my BIOS and they all give the exact same result. I have no idea what this is. Any help?