My Pi Won't Boot
If your Pi won't boot, check out this great reference thread from the Raspberry Pi Foundation forum:
IMPORTANT - The Pi Zero/Zero W boards do not have a power LED, only the green 'act' LED. This means that if your microSD card containing the operating system isn't configured correctly, the Pi Zero/Zero W will not boot, and without disk activity there's no indication that the board has power. People often assume that the Zero/Zero W board is DOA, when there is a problem with the configuration of the OS on the microSD card.
NOTES - For the Pi 4 ONLY:
Starting with the bootloader in the 2020-09-14 rpi-eeprom update, the 'act' green LED behaviour has changed. The green 'act' LED will stay solid instead of flickering at the beginning of the boot process and blink only to indicate a boot error. See the "boot failed" screen on HDMI0 (closest to the power port) for details about the failure.
Starting with the bootloader in the 2020-04-16 rpi-eeprom update, there is a new "boot failed" screen displayed on HDMI0 (closest to the power port) on boot failure. It provides technical details about the reasons why the Pi isn't booting. If you are curious and want to see the screen, try to boot your Pi4 without a micro-SD card!
Pi 4 bootloader release notes: https://github.com/raspberrypi/rpi-eeprom/blob/master/firmware/release-notes.md
To recover from Pi4 EEPROM corruption, follow the "Recovery" instructions on the Official Raspberry Pi Downloads page.