PC works over network but no video signal

I have a problem with a Dell Dimension 4500 at my office.  Came in this morning to find it
crashed to a blue screen (which suggested that it should be rebooted if the screen had not been seen
before - although I have no idea what the screen said exactly, as the PC was rebooted before I actually saw it). 

After reboot there is no video signal at all - no B&W bios boot-up VGA, no nothing, just blackness. 
However, the computer still runs - it is on a peer to peer network and I can still pull up files from
the machine over the network.  The monitor is fine and I have switched out video cards (slot is AGP 2.0
[4x]) but with no change.  Also tried using both analog and digital connections (both video cards have
both DVI and analog outputs) so I have also tried different cables as well - but nada. 

Any ideas?  Thanks.