Thank you for your very speedy reply.
I will resubmit a question if I cannot resolve the issue myself but it happened when I changed a rule in Apple mail (from the app) and then all of the mail in my inbox disappeared.
However the items (most of them) have suddenly shown up in the mailbox which the rule was targeting. The additional result is that many of the mail items had been archived but they all seem to have moved to this single mailbox. Nothing went to the trash and some of my rules disappeared so I am planning to delete the mail account locally and add it back as has been suggested in Remove or temporarily turn off email accounts in Mail on Mac - Apple Support
Thanks again for your help.