I replaced my modem and now the machine starts and the monitor light tins green, but the screen stays dark. Any thoughts.

Please provide more details about your setup. There are many Macs that came with the G5 processor. We need the following:

Mac model (usually listed somewhere on the Mac, be it an iMac, MacBook Pro, MacBook, etc.)
OS X version installed (10.4.11, 10.5.8, 10.6.5)
Make and model of modem installed


Thanks...