My Pi Won't Boot - Red Light Only
If you're seeing the red power LED light up, but no green 'act' LED indicating boot/processor activity, 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 processor 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.