Disappearing "From" field in Apple Mail
Open your preferences and go to "composing" amd change the setting for "Send new messages from:", close the preferences, quit mail, and then reset your preference to what you want.

If this doesn't work, make sure you don't have any Mail plugins installed.

Check the folder(s) if they exist:

$HOME/Library/Mail/V2/Bundles/
 $HOME/Library/Mail/Bundles/