Check the Archived Chats folder in Messenger
Search the conversation by the person’s name or keywords
Review your Facebook Download Your Information data
Download a copy of your Facebook data from Settings
Check email notifications for message previews
Ask the other person to resend the messages
Restore from a device backup if you have one
Check linked devices where Messenger may still be logged in
Look in your phone’s notification history if available
Contact Facebook Support if the account or messages were removed by mistake