Check the Archived Chats folder in Messenger
Use the Messenger search bar to look for the conversation or contact name
Review your Facebook Download Your Information archive
Open Facebook Settings and request a copy of your data
Check email notifications for message previews
Ask the other person in the chat to resend the messages
Check any linked devices where Messenger may still be logged in
Review device backups on your phone or computer
Check third-party backup apps if you use them
Contact Facebook Support if the account was compromised or the messages were lost due to an account issue