Does your monitor auto-scan for inputs, or do you need to manually select the input with a control button? If the signal dialog doesn't say which input it's looking at, and it doesn't auto select, you might need to specifically choose DVI or HDMI if the monitor is set to VGA.
If the monitor is definitely set to receive DVI and HDMI and not getting either, my next thought would be that the system might currently be configured to display only on VGA, which on that computer outputs only via the DVI-I connector. The blue VGA socket is disabled when a video card is fitted. The VGA output needs a DVI to VGA adapter, one of which was originally supplied with the unit:
If you notice in the link above in the video ports section: * VGA using included DVI-to-VGA adapter
This is the type of adapter you'd need to try (not a brand recommendation):
Edited by Platypus, 13 May 2015 - 09:05 AM.