Lost emails

In the Mail app, have you tapped on the account in the Sidebar and checked the Trash? If it is a 3rd party email address, have you checked the email provider's web site to see if the emails are there?

If you remember the old ID and password, go to iCloud.com/Mail using a browser, sign in with the old ID, and see if the emails are there. If they are, forward them to a current email address.