All my mailboxes disappeared on Catalina

Hi, my iMac is running on Catalina 10.15.4

All of a sudden, all my mailboxes disappeared. If I try to create a new one with a name, that I did previously, it says it already exists, but I can't see it. I know this has been asked on here before and I read a few posts, but nothing I've tried seems to work (f.e. go to view/show mailboxes - that option is not in my menu!). If I go to library/mail/mailboxes I can see a few there but not all of them. I tried to contact apple support and they are not available at the moment. I need the mailboxes back urgently. Any advise would be appreciated, thanks!