My S5 began restarting randomly a few days ago, saying "updating app 1 of 1." Today it will cycle through the start screen but never actually turn on. I figured it has something to do with the new software update. I went to the local verizon store, and they sent me home with instructions to plug my phone into my PC and run the software repair assistant.
Well, when I got home and plugged the phone in it would not engage with the PC, because the phone won't stay turned on. And since it won't turn on, I can't download the repair assistant software. And if I can't download thee repair assistant software, I will not be able to fix my phone.
First try doing a System Cache Clear if possible and then using the software repair assistant. If that fails to resolve your problem your next move would be a factory data reset. Follow for information and have a complete backup if doing the Factory Data Reset:
Try doing a "Wipe cache partition". This will clears the cache of your Samsung Galaxy S5 and should fix your first issue. The good thing about the Wipe cache is that no files are deleted.
Power down your S5
Press/hold volume up and home keys, then also press/hold power key
When phone starts to vibrates, release power key
When the system recovery menu displays then release volume up/home keys
Use volume keys to navigate to the wipe cache partition and press power key to select
When your finished, press power key to reboot your phone system.