The "FROM" choice has disappeared when sending mail.

The "FROM" selection has disappeared when sending mail so I cannot select any aliases, and all mail is now sent from my primary "ME" account.

Also the aliases are missing from the ACCOUNTS field in PREFERENCES, and I cannot reinstate them as Mail freezes when I try and press the Done key and have to quit Mail to clear it.

All aliases receive mail perfectly ok.

Grateful for any ideas.