Verify that it actually rebooted – if you can do a “net use” against the remote server, then connect to Event Viewer to see if it actually did reboot. I’ve seen plenty of Win2k3 servers that, when rebooted from an RDP session, don’t actually reboot but stop responding to RDP. If that’s the case, you can do a shutdown /i against the server or use some other out-of-band method of actually executing the reboot.
And if this is the case, in the future, don’t reboot from a regular RDP session. Reboot only from the /console or /admin connection, or use shutdown /i, or other remote methods.