Question
The long main branch of my 26 year old Cecil Brunner died suddenly this week.
All leaves are dry and dead. The shorter secondary branch is OK.  I have two
other large Brunners nearby that are pushing new growth as usual. The care is
the same for all.  It was very hot a week ago, but that's not unusual where I live.

Answer
Many area of the States  and other countries, have had nasty winters. This can cause dieback in  rose canes. However the problem doesn't show up as the rose can continue growing with the moisture that is left in the defected cane. But if you get a hot spell, which will put even healthy canes under stress, the damaged cane then suddenly dies. Your Cecil Brunner would have just slowly died but the heat wave and hasten the death of the cane. Cut the bush right back to clean normal growth and Cecil will bounce back for you.