New Outlook lost all emails in my created folders - how do I get them back?

Question

I recently got a new computer, and when I signed into Outlook it was the "new" version. I really don't like it, so I switched back over to "classic". However, despite having the folders I created showing up on the side tab under inbox, they are all empty. I sort and save important information I need to reference in the future there, so having that all gone is going to greatly impact my ability to do my job. How do I get the contents of my folders back?