why have mailboxes dissappeared?

This morning I found that all my mailboxes, other than those associated with a specific email had disappeared. I think all the emails I filed in those boxes are still there but I can’t see the mailboxes. For me this is a disaster since I file emails from different email addresses in these mailboxes. I haven’t changed any settings. Am running Big Sur. Where did my mailboxes go and how do I get them back?