This article will teach you how to unarchive messages in Facebook Messenger by sending a new message in an archived conversation.
Step #1. Open the Facebook Messenger app
This is the blue speech bubble with a white lightning bolt in it.
Step #2. Tap on the search bar
This is at the top of the screen.
Step 3. Type in a person's name
This should be the name of the person whose conversation you previously archived.
Step #4. Tap on the person's name
This will open a chat window with the archived conversation.
Step 5. Type a new message
Step 6. Tap on the blue Send button
It's to the right of the message box and looks like a blue paper airplane, or the word "Send" in blue. This will send the recipient a new message, and the conversation will automatically be moved from the 'Archived' folder to your inbox.