Outlook 2000 – disappearing ‘inbox’ mail

Since last night, I suddenly started to lose emails from my inbox once I read them i.e. read mails simply disappears from the inbox. Previously, all read emails stayed in the inbox. I have lost a number of important emails this way. Please could someone advise:
Where to find them?
How to retrieve them?
Any Outlook settings that have to be changed that I may have inadvertently changed.
There is no other user account using Outlook 2000.
Thanks