After you delete a conversation it will no longer be available in Recent conversations. It will move to Trash on desktop.
If you archived your conversation from your Android it will be still available on Archived.
To restore your conversation:
1. Go to Messages on the vertical menu to the left side of the page.
2. Choose the Trash tab.
3. Hover over the conversation you want to restore and click on the Restore icon.
OR Click on the conversation you want to restore then click the Restore icon on the top right corner of the page.
4. You conversation will be moved to Recent.
If you want to restore the conversation you just archived simply click on Undo:
If you want to restore the conversation you delete before:
1. Tap on the hamburger menu icon.
2. Tap on Discussions.
3. Tap on Archived on the top right of the screen.
4. Choose the conversation you want to unarchive and swipe it to the right.
5. If you want to archive it again just click on Undo.