Update: It may still be wonky, and we still don't have an explanation for what happened, but Steam is now back in operation, store and all. We'll keep an eye on things, and update again if the situation changes.
It's not just you: Steam is down. As far as we can tell, it's down worldwide (a contact in Germany confirmed that it's down there), both the store and services, meaning that some of your single-player games won't work unless you go into offline mode. (I just tried to fire up Fallout 4 and it's a no-go; Escape Goat 2, on other hand, works fine, albeit without access to cloud saves.)
We don't know what's going on at this point, but I've emailed Valve for more information and will update when the situation changes. Hopefully, we'll be able to dive back into the Steam Winter Sale soon.
Keep an eye on steamstat.us for an unofficial check of Steam's many servers.