To prevent accidentally accepting writes, our default my.cnf settings tell MySQL to always start up read-only. If the primary MySQL gets restarted, it will thus come back read-only until you intervene to confirm that it should accept writes. You can use the SetReadWrite command to do that.
However, usually if something unexpected happens to the primary, it's better to reparent to a different replica with EmergencyReparentShard. If you need to do planned maintenance on the primary, it's best to first reparent to another replica with PlannedReparentShard.