Hi leeann47,
welcome to the Apple (user-to-user) Community.
Re: "Mailbox disappeared : would like to re-find it as it had important correspondence."
It is not that difficult to accidentally "swipe" a mailbox into another mailbox (eg: into the mailbox above or below it).
Any Maiboxes that contain other mailboxes have down arrow (v) when closed or right facing arrow (>) when open.
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If needed: You can try searching for the important correspondence :
In Mail on your Mac, search for emails using your own words or using Mail suggestions and search filters....
Search for emails in Mail on Mac - Apple Support
and
If searching doesn’t find the right emails in Mail on Mac - Apple Support
Apple support also have search tips in the linked article: Narrow your search results on Mac - Apple Support
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More tips, (including for Time Machine): If emails appear to be missing in Mail on Mac - Apple Support
or
How to: Rebuild mailboxes in Mail on Mac - Apple Support
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More help / info. / support / contact:
Mail support website:
Mail - Official Apple Support
All the best :-)
