I had the same problem with my S5. One moment it was working, the next it was mysteriously caught in a boot loop, then it was totally inoperable. The phone was 2 and a half years, old, no damage whatsoever. I brought in to Verizon and the first thing they suggested was to buy a new phone, without looking at mine. Next, they did look at it and said it was either a software or hardware issue and told me to go to a phone repair place. The repair shop had my phone for three days and told me they had no idea what was wrong. It would not turn on, even with a new battery. I lost all of my contacts and a year's worth of photos.
I activated my son's S5 and transferred my number. This time my phone lasted six days before it started overheating, and again I lost my photos (after Verizon supposedly set up my backup to the Cloud). I am thoroughly disgusted!