Well, after some poking around I finally solved the problem....
1. The onboard nic was not dead, just a little confused.
2. The problem was in /etc/fstab in this line
I found these errors in the logs -
CIFS VFS: cifs_mount failed w/return code = -6
When I commented out the line above in /etc/fstab, everything started working again.
The network mount was for a NAS that turned out not to work very well, so I removed it from the network and must have forgotten to comment out that line in /etc/fstab.
I hate it when things like that come back to haunt you years later.....
Mark