Sounds like it's booting to recovery mode. You might be able to manually flash a new boot loader using odin. I did it a few years ago with my Epic 4G (a Galaxy S1). Odin requires a windows box. I did it from an XP VM on my Mac.
-Heath Borders
Sent from my Nexus 5
If you ever rooted, you won't be able to install a stock ROM or OTA. You have to un-root first. Usually there are YouTube videos explaining how.
-Heath Borders
Sent from my Nexus 5