PowerMac G5 won't see Ethernet cable?

Go to the configuration settings for the modem/router and make sure it says connected . try disconnecting it from the system preferences menu also

Swap out the cable. That could have gone bad. As @sandystrachan suggested, check the connection at whatever switch or router you are plugged into. If you have IT folks take care of that, ask them to check it.

I have swapped out the cables and the network guys have checked the port. All of that seems to be good.

Go into System Preferences > Network > Ethernet. Make sure the ‘Configure’ drop-down menu says ‘Using DHCP’ (rather than Manually, Off, DHCP with Manual Address etc).

Click the ‘Advanced’ button in the bottom right and in the resulting window, click ‘Renew DHCP Lease’. A lot of the time this will sort out network problems.

Benseven’s tip may work if your office uses DHCP and not a fixed address system. You can also toss your Network Preferences and see if that helps.

You may have a flaky Ethernet card; you may need to replace it. Here are some tips on testing if your card is the problem.