A file server at the University of North Carolina
It went missing in 1997. Finally in 2001 IT workers started tracking down the server by meticulously following the cable that connected it to the network. They literally found it behind a drywall, which maintenance workers had but in by mistake.
The server, which is running Novell, continued to function perfectly for the 4 years it was missing.
Taking from an article on TechWeb dated April 9, 2001