Hi @GClare sorry about the delay getting back to you, 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?
All emails prior to 18 January have disappeared. The link only deals with those lost in the last 7 days.
I did not delete these emails. Why would they disappear from my account? There are important documents in there.
Do you only use webmail, or other devices to connect to email?
Do you have any filters set up, or move emails between folders?
If you only use webmail, you should seriously consider a backup routine of using a mail client program or app that downloads the emails to the device (but leave messages on the server).