How did I unarchive a conversation in facebook massenger?

Open your Facebook messenger and browse to your messages. You will find a search magnifier in the right corner of your display. Make a click on that. Select the archived messages from there. The option of “Unarchive message” will be available beside each archived message. Click on un-archive and confirm your action.

By doing this you can unarchive all the messages that you have archived previously on Facebook messenger.