iCloud emails disappearing

This is either a glitch, soemone in your account, or a hacker. Fixes: update devices (may work), restart devices (may work), change Apple ID password, go to iCloud mail on the web and click gear icon and delete any rules that are there, turn on protect mail activity (settings, mail, privacy protection) on all devices.

Look up your mail settings , enter correct credentials and when verified Mail – Official Apple Support

Remove the mail account and once again add it Add and remove email accounts on iPad - Apple Support

Note : installing third party mail apps / softwares will corrupt the inbuilt applications How to delete apps - Apple Support ( remove them if installed ) , don't use vpn services .