If an enthusiast attempted to install a custom ROM (such as CyanogenMod or an early version of LineageOS) and accidentally corrupted the phone's partition table or boot loop, standard recovery methods might fail. Because an RUU executes at a low level via Fastboot, it can completely wipe the damaged system files, re-partition the flash storage, and cleanly reinstall the operating system, breathing life back into a "bricked" phone. 2. Reverting to Factory Configuration

The current showing on your device's bootloader screen. Whether the top line of that screen reads S-ON or S-OFF .