We have a Windows 2003 server with 2 NICs; one NIC is for our client access and general network connectivity. The second NIC is used for private communication between other servers. The second NIC doesn't have a gateway defined, nor has it ever. The first NIC has a gateway and DNS servers defined.
This server can ping everything on our network; however no clients or other servers can ping this server. We have disabled AV protection and confirmed there are no firewalls blocking traffic.
This server, and other servers similarly configured, work just fine. We did not suffer from any power outage or anything out of the norm so we don't have a cause for this incident.