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 lock topic, the first can be in a way users will be able to still unlock it if they want, the second can be a in a way users will not be able to override it. Although, I am only guessing, moderators and admin will have the best answer to this in order to avoid confusion.