From my experience it is often moisture that causes these types of problems...
After my girlfriend had this problem with her Note 4 this afternoon, I did some googling and ended up here.
My fix was to turn the oven on...
Getting the temp to about 60 degrees Celsius...
Take the back off the Note 4 and battery out
Remove the sim and external SD card
Place the phone in the oven for 30mins
After 30 mins remove and allow to cool
Put it all back together and presto
Working fine now!!!!!
The Girlfriend is HAPPY!!! :)
Regards,
The Boyfriend.