How do I restore missing mailboxes and emails on my MacBook Air?

Hi

yesterday suddenly and cannot think if and how I caused it but all my mailboxes disappeared along with their mail on my MacBook Air. I was only left with Inbox, flagged, drafts and sent plus 2 smart mailboxes today and guardian?? which I don't use, even bin and junk had gone along with 12 others. My iPhone and iPad are still as usual. I tried log out of mail and restart and log back in but still the same. I saw 'synchronise with iCloud' but don't want to risk losing them on the other machines. Any clues?

[Re-Titled by Moderator]

Original Title: Mail