Email alias disappeared from my iPhone after update

If you are referring to a Hide My Email address, visit the Hide My Email list in iCloud settings and look for a cog icon at the bottom left of the email list. Click that and choose "Show Inactive Addresses". You can use the "Reactivate Address" button to add your alias back.

Do you possibly have more than one Apple Account and were logged into iCloud with the other one?

If that's not it, then hopefully you can use whatever account recovery process is available from your "open AI program" to gain account access and change your account email.