EDIT: the outage appears to be resolved.
The outage is a steam issue.
Please note we get no more information from steam about these things than you do.
We only figured this out by seeing it affecting other games. This post is being written 3am my time time, why steam don’t announce this is something only they can answer.
If you go visit the Project Zomboid forums you can see people having the same issue.
That steam don’t announce these issues is very problematic for us, because we get the negative reviews (we already have now for this outage) for something which they cause.
It appears to line up with a cloudflare outage, so you can monitor the status there. Once this is resolved, I recommend you restart your computer
Dean Hall, Irritated and tired CEO