Missing Mailbox Folders in Apple Mail

After upgrading to Big Sur, some of my mailboxes (folders) and sub-mailboxes have disappeared. The emails are still there, as I can search for emails that are in them and they show up, with the name of the missing mailbox shown to the side. But that mailbox is no longer in the list of mailboxes to the left side.

This has happened in the past with different OS upgrades. It is very selective. I have 75 or more mailbox folders to sort my business mail, and only a few disappear every year or so. How can I rediscover and retrieve them?