offline
Not or no longer connected to an Internet network. I'll be offline for a few hours while I fly to LA. The power finally came back on in the office, but the whole building was still offline for several hours after that.
off-line
not connected to a computer, by direct connection or via the telephone system, etc. (Compare this with online.) When he's off-line he's sort of lost. The system was off-line all day, so we could get nothing done.