iCloud email disappeared

Back up your device Back up iPhone - Apple Support then erase the device Erase iPhone - Apple Support

Follow the articles Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

Your question is not clear which device is being used Mac / iPad / iPhone and what are the versions , in Mac is iCloud account configured with apple mail application .