Update: It's still a bit shaky, but Steam is slowly coming back online now.
Expect it to be a bit intermittent as everyone in the world tries to log in at the same time, but Valve's service is slowly recovering now.
Have a read of our list of the best games of 2016, which you can't currently play, because Steam is dead.
Update: A self-proclaimed "cyber terrorism" group have claimed responsibility for the Steam outage.
A group who call themselves Phantom Squad have said they launched a "major DDoS attack" on Valve's gaming service. We have emailed them to see if they can provide proof and to find out their motives. Their Twitter account has since been suspended.
Back in November, the group claimed responsibility for a short Steam outage, along with one in Rainbow Six Siege. At the same time, they made threats against Steam, Microsoft and Sony for the holidays.
Original: It’s either the end of the world, or Valve are having some big server issues, as Steam has been down for around an hour now. Steam being down during a sale is nothing new, but this isn’t even intermittent - it just plain doesn’t work at all.
Not being able to play online is expected in times like this, but currently some players can’t even access their offline games - we've tried it with various titles, and one of our computers straight up can't even start Steam. We'd recommend leaving the program open if you already have it. Everything is on lockdown and it’s kinda bizarre.
Even Steam developers can't access their sales stats, as SteamWorks is also unresponsive.
— Steve Stewart (@ThatDarnSteve) December 23, 2016
We’ve reached out to Valve for comment, but we’re not expecting to hear back any time soon. I imagine they’re being inundated with emails right now. Remember last Christmas on Steam when people could see each other’s bank details for a little bit? That was fun wasn’t it?
Unofficial status tracking sits Steam Stats is a good place to track the current status of things, but we will keep you updated if there’s any change. It looks like there are quite a few people hoping for answers.
— Pavel Djundik (@thexpaw) December 23, 2016