As long as you have already logged in successfully once, the client is designed to switch automatically to offline mode if you do not have an active Internet connection or cannot connect to the game servers. Also, if you wish to force the client to start directly in offline mode, you can tick the Always start Uplay in offline mode box in the client Settings, or via the main menu by selecting Go offline.
While in Offline mode, most features will be unavailable, but you can start your game from the Games tab as usual. Almost all games in our current catalogue can be played offline after they have been activated, providing they do not have any always online requirements, and your solo campaign will resume from the latest local save.
Once in Offline mode you can switch back to Online mode at any time by clicking Go online in the client's main menu.