After the moderator or administrator closes the topic, the creator of the topic can not unlock it. If the creator tries to unlock the topic, he will receive a message:
The question will be good to be answered by one of our moderators, very possible moderators can have two ways to unlock topic, the first can be in a way users will be able to still unlock it if they want, the second the be a in a way users will not be able to override it.