I know how you feel Cheryl. I had similar thing happen to my very first plumeria, Trimark. So the lady that gave it to me advised me not to take any sign of rotting lightly because it could spread fast.
If you suspect that the top is starting to rot, I would suggest you cut the top that shows signs of rot until you see fresh white sap. Then cover the exposed cut until you can find a pruning seal. Hopefully you get sun soon so the top part can completely dry or heal.
I was able to stop the rot on mine and my Trimark is now 4 years old and the one that blooms for me every year and has given me a lot of cuttings.
