@apz So, I think there are a few things that can happen here.
As you mentioned, content, actors, and so one that had been created and changed during that time would now be missing, so requests for them would fail. I think that's pretty survivable.
There are other parts of the ecosystem that aren't quite as robust. Usually, for example, if an activity is sent from one server to another, and accepted, the sending server updates its state marking that activity as delivered.