Sounds like I want to know why my neighbour died. He only had a last breath and that was the end. Won't respond to anything.
As an electronic repair engineer, I always used electronic measurement tools, to diagnose defective apparatus. It took some time, to exclude the working parts and to find the real culprit. It won't be possible to tell you what is wrong, with your phone unless some technician is capable to view it. Becoming hot, could mean the battery is still charged, but perhaps it can't deliver any power. But there could be lots of other things, like small shortages in the wiring or even a break somewhere. Of some oxidation causing a to high resistance. As said lots of things.
By the way, My neighbour died of a hart failure, but it took the medical examiner 2 hours before he was sure.
please for your phone visit a Sony certified repair centre, Before you let them investigate, ask what it will cost you, because it could be expensive. If it is still in warranty, no need to ask. Unless the phone was in the water once, because they will find this and it is excluded from warranty.
on Sep 06, 2014