Unfortunately this is one of the risks of federation. It’s alleviated on e.g. Mastodon as you must have more than one admin.

The last time Tom was not able to respond I asked about what happens. I was told that basically there’s no provision for this situation so all you can do is use a third party tool to migrate your account elsewhere. I was told that eventually hosting will fail or a manual upgrade will be needed then the instance will be no more.