Some Power Computing models did not come with Ethernet built in (as all Apple machines do). Odds are you have a 3rd-party PCI Ethernet card and the upgrade wiped out the drivers for it. The drivers were probably included on a floppy when you got the machine (ours were). Conversely, you an look inside the machine to see what kind of card you have and contact the company for the drivers. (If you have some other way of getting online, it's probably easiest to download the latest drivers directly from their site.)
Hope this helps,
Jeff