It does sound like a watering issue. Either water was not reaching the top of the plant because the roots were too dry to take it up or perhaps the rots rotted from too much water. They need to be watered well and drain well. Do you take it out of that outer container and let it drain for a while after a thorough watering? I know Logee's sells those avocado trees but they are really not houseplants. It is very hard to keep something like that in CT. I would prune off the dead part and keep my fingers crossed that the remainder will live.
Good luck with it.
