Update: It seems that the Steam is back online. We still don’t know what caused the outage but we’ll let you know as soon as we do.
A large number of Steam users are reporting that they receive “No Connection” errors when they try to access its services.
Some players are able to start Steam client in offline mode and play single player games with SteamWorks features disabled, but others were not as lucky as their Steam clients refused to launch at all. At the moment, there is no explanation for the different behavior or for the service disruption itself.
Meanwhile, it seems that the Steam Store is not affected.