Can't open windows, computer screen has

Question

When I switch the computer on, a message: ATTENTION NO VIDEO INPUT floats around a black screen and I cannot get anything past this despite switching it off and on repeatedly. This first started while on a banking site and the screen went blank and after a while came back on and then blanked out again and then I got this message. I have checked all the cables and everything seems to be plugged in properly.
I am using Windows XP (well, I was!!)