It must have been up at some point then.

Federation is a hub and spoke model. Each community is a hub, hosted on one instance.

When you subscribe to a remote community, the community checks its list of subscribers and sees it there's anyone else from your instance. If not, it adds your instance.

Every time someone votes, posts, or comments in the community the host instance sends this info to every instance on that community's list.

If the community's instance is offline, other instances can still see old posts and comments, as a local copy is kept.

Users can even continue posting, commenting, voting, etc. like normal. However since your instance cannot tell the host instance of these interactions, only your fellow users on your instance will see this. Your instance will try to resend the data occasionally. There is no path for those interactions to make their way to any other instance. The fediverse is decentralized in many ways, but communities are not decentralized.

It's possible the instance came online briefly while the admins were testing and restarting things.

The same happens with defederation. At first it seems like nothing happened, you can go to the defederated instance's communities and browse content, comment, etc. but nothing new comes in and nobody outside your instance sees anything new you do.

Source: am instance admin