bootloader - Raspberry PI 4 not booting, ACT LED blinks 4 times, boot recovery won't help - Raspberry Pi Stack Exchange
My Raspberry Pi won't boot into Raspbian any more? - Raspberry Pi Stack Exchange
Raspberry Pi 4 8Gb not booting - Raspberry Pi - Core Electronics Forum
7 Causes for a Raspberry Pi That Won't Boot (And How to Fix Them)
RPi 3b doesn't boot from sd card. Without sd card, both ethernet light up, even with sd card inserted too. ACT led wont light up. 3.3v rail works. : r/RASPBERRY_PI_PROJECTS