Welcome to this user forum.
If you're on the BT Yahoo mail platform you can recover deleted/missing emails by following the guidance on this link. BT Yahoo Mail - I’ve lost or deleted some of my emails. Can I recover them?
That will depend on which email system you are on.
If you are on the BTYahoo email system have a look at this link
If you are on the BTMail system you can not recover deleted emails that were in the emptied trash folder.