Your graphics card probably fried out. Did you notice any strange blue lines before?
Mine fried, and it still had a blackscreen with white numbers.
I had white numbers during start up screen, they weren't suppose to be there.
I actually think I've had your problem. Check the wires connecting your Monitor to your Computer, maybe its loose or something. Hopefully that might help.
Also, my computer was making an error code sequence of some sort.The guy who looked at it told me it was my Graphics Card, and it got fried because the fan bearings got all forgeted up.I've got a heavy feeling it could be the monitor. But you don't have a spare :C
He said that he had tried a spare monitor after finding one, and it would turn on but wouldn't show anything.
Is the case for your tower hot?
Crap,no extra monitors around. Obviously, if the other one worked, it would be broked monitor,lets say the extra monitor doesnt work.