My DHCP is set up to hand out my internal DNS server as the primary DNS to
all workstations at log in. If the internal DNS server cannot resolve the IP
address, then it passes to an external DNS server. This has been working
flawlessly for over 7 years. Suddenly, in the middle of the day, this stops
working and no one can access any external web sites. I checked all external
DNS servers and they were working. It just seems that my DNS server is not
passing the unresolved IP addresses on. I restarted the DNS server, but no
change. I changed DHCp to start handing out two external DNS servers as
Secondary and Tertiary DNS and everything started working again.

Any ideas why this would stop working suddenly?