If it disappears, you're right. If it's "banned" (not a thing, assuming you mean defederated) then you can't move to one of the instances which have defederated it. Any followers from an instance which has defederated yours will also be lost, not because you can't move them but because the block on their instance will automatically cause them to unfollow you.

The solution to this is to move to an instance which hasn't defederated from the one you're on. Worst case, setting up your own.