Sending, receiving, and organizing email in Outlook.com
Hi. I'm Brian.
Are you speaking of a personal Microsoft account or a business account? Alas, if it's a personal account, if you do not make backups for yourself using a mail client program and you lose the contents of your mailbox, it's gone. Microsoft does not make backups for you.
Typically, if you delete a folder or any messages, those are moved to the Deleted Items folder and you can recover them from that folder if it has been less than 30 days since they were deleted. A deleted folder will cause a > to appear next to the Deleted Items folder in the navigation pane and if you click that, the folder should appear. You can then click-and-drag that folder back to the top level or to the folder from which you deleted it originally.
After those 30 days, you should be able to open the Deleted Items folder and click the link "Recover items deleted from this folder" link at the top of the message list. Your deleted messages that have been removed from Deleted Items should appear there and you can restore them. After that extra 30 days, however, they are gone for good unless you had made a backup of them.