We're going to need some additional information about the network... what kind of hardware is being run? How exactly did the network get screwed up... stuff like that. Without said information we really have no where to start.
Ok, well something had to have changed between the time it was working and the time it was not working. Any new hardware, new devices on the network anything like that? Also I hate to ask the obvious but have you tried rebooting everything?
Checked yer router firewall? Has there been a power outage that would cause stuff like forwarded port settings to become lost?
Ok, but it wouldn't hurt to check. Some games/network configs require a port to be forwarded.
Have you checked your firewalls?
Does your friend use zonealarm?