Only instances with a “.ml” at the end of the name may or may not be affected. Lemmy is a collection of instances so the loss of a few will not cripple the whole thing. Content over the whole is not greatly affected.
If your home log-in instance is one that’s affected, you’ll have to find a new one. You’ll know right away because the instance will be unreachable. Not a big deal, last time I looked there was over 1200 instances to chose from.
Another consideration is any communities living on an affected instance may have issues. All communities are common to Lemmy, but each originates from a particular instance. We’ve not yet seen a major instance go down so I don’t know how Lemmy deals with communities getting orphaned like that.