Once a message is deleted, it is removed from Facebook’s servers and cannot be retrieved. However, if you have enabled message backup on Facebook, you can still retrieve the deleted messages from the backup files. Here are the steps to check for backup files:

1. Log in to your Facebook account and click on the gear icon at the top right corner.
2. Select “Settings & Privacy” and click on “Settings.”
3. Click on “Your Facebook Information” on the left-hand side.
4. Click on “Download Your Information.”
5. Select the data range, format, and media quality for the backup files.
6. Select “Messages” from the list of options and click on “Create File.”
7. Facebook will prepare the backup files and notify you when they are ready for download.

Once you have downloaded the backup files, you can use third-party software to extract the deleted messages. However, keep in mind that this is not an official method and may violate Facebook’s terms of service. Additionally, third-party software can potentially harm your computer, so use caution when downloading and installing any software.