Samsung Galaxy S5 stuck in bootloop and how can I fix it?

[Android Central Question]

Since last night, my phone has been stuck in a bootloop and restarting endlessly, making it completely unusable. When the Samsung Galaxy logo shows up, there's an unlocked padlock with the word custom, and I don't think that was there before. I've tried some solutions, but none of them work so far.

 

[Rukbat]

The system got corrupted, About the only way out now is to flash a stock ROM to it. (And you'll lose anything not backed up,) See [Samsung] How to flash Stock ROM via ODIN.