Here are five methods that could help you retrieve those messages:

Check your archived conversations

If you archived the messages instead of deleting them, you can easily recover them, as Facebook always keeps your archived chats.

1. Open the Messenger desktop, web, or mobile app.

2. From the chats section of the app, type the name of the person you were talking to in the search bar. 

3. Then simply click or tap their name to open the conversation and view everything.

Ask the recipient for a copy, or screenshot, of the message or conversation

When you delete a message, it only happens on your end. The person you're talking to will still be able to see it, so they can help you fill in the gap if they didn't delete them on their end as well.

Check if your Facebook messages were sent to your email

This only works if you have all email notifications enabled for your account. But if it's enabled, you should be able to search for the conversation or message within your email account. 

Use Android file manager

If you're using an Android device and you have Messenger installed, your device could have those deleted messages in the app's cache. You can access it using any Android file manager, and if you don't have one installed, you can find plenty of them on the App Store.

1. Open the file manager app on your Android device.

2. Navigate to the Internal Storage > com.facebook.katana > fb_temp

3. Here you'll find messages from your recent history, and with any luck, your deleted ones will be among them.

Quick tip: If you're using an iOS device, you can't recover your deleted Facebook messages using any file app — however, you can download your messages by backing them up beforehand.

Download your Facebook information

Backing up your Facebook information, including your messages, is a good idea if you don't ever want to lose any of it. 

1. Using the desktop app or browser version, go to your Facebook settings.

You can also access these settings in the Messenger app on iOS.

2. Within the Your Facebook Information tab on the left sidebar, click Download your information.

3. Under the Select file options heading, make sure you've selected a date range in the last dropdown.

4. To only get your messages, click Deselect All under the Select information to download heading and recheck the box next to Messages.

5. Scroll all the way down to the Start your download heading and click Request a download. Facebook will email you the file once it's ready for viewing.