If a moderator locked your topic, you can unlock it.
If an admin locked your topic, you can not unlock it. I only don't know a global moderator has same power like forum admins in locking a topic.
Do not mislead people
If a moderator has blocked a topic, then the author of the topic will not be able to unblock it
An admin can also block a topic, but in practice no one will even remember the last time he did this