Mailboxes disappeared - can't re-add!

I just went to my late 2013 MacBook Pro, running Mojave 10.14.4 to look at an old email from 2018 that I need to refer to. When I opened the Mail app, there are no mailboxes showing and it was asking me to set up mailboxes, as if I had just installed Mail. Most menu options are greyed out, including the Import Mailboxes option. I cannot even add a mailbox - it lets me go through the process (eg entering my AppleID credentials to set up an icloud mailbox) but then just returns to the start, without setting it up. The only option that works is Quit!

I can see mailboxes stored on the hard drive under username/Library/Mail/V6 but I have no idea how to restore them or even identify which ones they are, as I have several mailboxes usually.

Hoping someone can help!

Thanks in advance.