having just checked by plugging a spare vga card into a monitor, the `no signal` sign appears when the lead is removed, so no actual signal is present as such, just a matter of checking for ground signals or something.
do you get post beeps if you remove the video card and try?
have you got an old pci vga card to try instead?
i take it the board has built in vga?
personally i wouldnt be plugging/unplugging vga with the machine on - did you switch off inbetween?