Cannot Ping Server

I have a Dell PowerEdge 840 that has Small Business Server 2003 on it. The server was rebooted Friday night and ever since, nothing has been able to ping or connect to the server. The issue does not appear to be hardware related, since I switched this over to the 2nd NIC on the server and it still will not ping. DNS and DHCP stopped answering requests. The server will connect to the internet, so it has a physical connection that is working. The firewall has been disabled, and receives an ipnat.sys error when trying to enable it. The event viewer is basically clean. Server was up for over a month prior to the reboot, but it seemed to have a memory leak that was causing it to use an excessive amount of memory.(which has gone away since the reboot.) It is still emailing performance reports, but all DHCP client computers can no longer connect. Static computers can connect to the internet as long as external DNS servers are entered.