Reseat RAM (take out each stick and put them back in again). If there's still the red light when you power up, use only one stick, if it works then add another.
And perhaps look in the motherboard manual to see if the red light means anything specific.
And some questions just in case:
You have attached the CPU heatsink correctly?
Have you tried reseating the CPU heatsink?
Did you put thermal paste on the CPU/heatsink?
You have the 4- or 8-pin CPU power connector plugged in?
You have the 24-pin motherboard power cable plugged in?
You have the GPU power cable(s) plugged in?
Have you tried reseating the GPU?