So it happened with a push notification from AT&T indicating an update that needs to install. After finishing installation, my mobile phone re-start and prompts me with "encryption password" since I have my phone password encrypted. But the flash light just flashes and re-start my machine over and over until I took off my battery. Does anyone run into this issue before and hopefully have a solution to this kind of issue?
So I tried @Sjharrison's way to press home,power and volume button at the same time to put phone in recovery mode. And it gives me the option to safeboot my cell phone and I chose to delete the cache files. It automatically reboot and show progress on automatically installation. Then my cell phone works as before. Kudos on @Sjharrison!