Lost SENT Folder

with the mailbox sidebar opened, you can try and hover your cursor over the word "favourites" and click the "plus sign" that appears.

and then select "sent" for the "Mailbox to Add" field.

Is it possible that you accidentally dragged the Sent folder into another unexpected area in the list? Another idea to restore it, is to remove this email account from Mail, then set it up again using Mail, Settings, Accounts to remove and then to add it back after it's removed.

you're welcome. :)