what will happen if a Lemmy instance goes down?

I'm still pretty new but I believe federation works like syncing. You can see that at Beehaw, they defederated Lemmyworld so any communities from lemmyworld are still visible, but no longer updated.

Edit: I should mention it's not exactly like syncing, because federation won't sync backwards in time (for example is A defederated from B for three days, when they refederate they will see new posts but not what they missed in those three days.)

But if they stay federated then the copy should be mostly identical in terms of content.