Have you upgraded anything on the G5 lately?
Do you have another computer to try the display on, this will determine if it's the G5 or the display?
Have you tried running in safe-boot mode holding down the shift key, or starting from the System CD/DVD?
When you mentioned reconnecting everything, did you try reseating the video card. Make sure you ground yourself before doing this so you don't short-circuit something by touching the metal case.
It could be the video card has died.