Just last week I was fixing a computer client said was a problem reading email (this usually means there's no network connectivity at all).
Went there, and saw that dhcp was not giving address, the adapter was 0.0.0.0. So I checked the crosspatch-closet, changed ports in switchs, changed cables, called the networking department whether we'd ran out of ips on dhcp, spanked the computer.
Until I realized there was another nic in the computer, along with the integrated one, the integrated was not working. So I put the cable in the other nic, and whee, everything works.
This all took about an hour.
Felt a bit stupid