Some games may not be accesed in offline mode on steam, they are suppose to be but tend to "bugg"!
I'd send a mail to steam customers service to easly get an answer, cause if its a bugg then they will tell you if they inted to fix it anytime soon!
But have you tried this with no internet at all? As in steam was started or running in offline mode aswell? (Might be a REALLY stupid question but anyway).