Update: As of about 11:00 AM Pacific Time Steam appears to be back up, though it is clearly running slow. There have been claims on Twitter that Steam was, in fact, being hacked, though due to the anonymity of the internet it is obviously difficult to substantiate such claims.
From Earlier: Steam's big Winter sale started yesterday but if you want to take advantage of all the killer deals, you'll have to wait. The entire service is down right now. The storefront is down and game servers and communities are all unresponsive. The exact cause of the outage is unknown and as such, we have no idea when things will be back up.
The obvious culprit is the Steam Winter Sale which began yesterday. Dozens if not hundreds of games are on sale which is going to draw huge traffic numbers of people who grab ahold of popular games and sometimes insane discounts. That can certainly cause the service to crash. However, in the past, hacking groups have used the holidays as a time to target gaming networks like Xbox Live and PSN with DDoS attacks. It's possible that 2016 is Steam's turn for such an attack. We'll keep you updated here as details are revealed.
Two years ago the hacker collective known as Lizard Squad hacked both Xbox Live and the PlayStation Network simultaneously, causing outages for both services at Christmas, just as millions of gamers were getting new games to play as Christmas gifts. Xbox Live recovered faster than the PSN was down for multiple days following the attack.
Later, it was reported that the DDoS attack was only brought to an end when Kim DotCom offered the group premium memberships to Mega in exchange for stopping the attack, and also never doing it again. Since Steam wasn't part of that attack, one supposes they wouldn't have been part of that deal.
Of course, perhaps being down completely is preferable to some of the alternatives. Last Christmas Steam went completely haywire, resulting in people logging into Steam only to discover that they had access to other people's accounts. The issues lasted for most of Christmas Day.
Of course, there are also other theories as for the reason for the outage that are preferable to any of these
We'll update this article if and when any changes to Steam take place. Hopefully, this will just be a temporary stall so that we can all get back to spending money on games we'll never play just because they're cheap. Who knows, maybe peo0ple are just really excited about voting for the Steam Awards.