get the latest version of that pst-file from your backup, and import those missing folders from there to your current Outlook-database (using File>Import).
If you don't have a backup, you've lost your data. That's one of the rules of the game: you will (sooner or later) lose what you don't backup.

By the way: what do you mean with ''I was only prompted to delete that certain folder''. I never saw a prompt/request to delete a folder in Outlook. I just select the folder, and then use either right-click + delete or the delete-key. Both of these operations move the whole folder to the Deleted Items folder, and I can move them back as long as I didn't close Outlook.
Can you explain that?