Check the archived chats in Messenger
Search the conversation name in Messenger
Review your Facebook download data
Go to Facebook Settings and request a copy of your information
Open the downloaded Messages folder in the data archive
Check email notifications if message alerts were enabled
Ask the other person in the chat to resend the messages
Check connected devices where Messenger may still be logged in
Look in device backups if your phone backs up app data
Use third-party recovery tools only if they are trusted and safe