Using Classic Outlook on Windows for personal email, calendar, and contact management
If the messages are missing only from Classic Outlook, I’d first check whether they still appear in Outlook on the web. If they’re missing there as well, check Deleted Items and then Recover Deleted Items From Server in Classic Outlook. Microsoft notes that recovery availability depends on the account type and retention period, so checking the server copy first can help determine whether the messages are actually deleted or just missing from the local Outlook view.