Someone closed the valve at Valve.
Update 3:00PM ET — The outage appears to be largely resolved as users are reporting their service is back online. That means you should no longer have issues browsing the Steam store, opening up your library, and playing games with friends.
Update 1:30PM ET — Services across the U.S. appear to be in working order again. Many European countries are still showing significant outages. Steam has yet to provide an official report regarding this outage. An updated map is located below.
Popular PC gaming service Steam seems to be having some issues today, January 30, as users across the globe are reporting connection problems to the service. Downdetector is only showing significant outages across Europe, but users throughout the U.S. are also reporting problems connecting.
Much isn't known about the nature of the outage, but users began reporting issues somewhere around noon ET. If you're having problems connecting to the Steam store or playing games on the service, know that you're not alone, and Valve should be working on the issue.