Hi,
This has happened to a few other users as well. Typically when I see an issue with Autodesk manage portal, I recommend either logging in from a different browser (Chrome seems to work best) or if you're using your preferred browser and it recently changed, then clear the cache for the browser and login fresh.
This link provides a shortcut for clearing your cache: Shortcut for Clearing Browser Cache
If that doesn't work, since this is an Autodesk Account issue you should open a ticket directly with Autodesk as they can emulate your account and verify if Classic User Management still shows on their end, which will confirm that it's a browser issue.