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 someone intervene to confirm that it should accept writes.
If VTOrc is running, then it will take care of converting the primary to read-write mode.
However, to fix manually, you can use the SetReadWrite command to do that. 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.