What is your mobo?
IMHO disassemble the computer. Start with the very basics.
Outside of the case on a cardboard box place the mobo (mobo box works great)
Re seat CPU.
With the mobo, single stick of ram, GPU (and the CPU of course) see if it'll post then?
If it does not, try the placing your single stick of ram in the next slot. Keep trying all the slots.
IF it still doesn't post, repeat the with other stick of ram. It is unlikely both sticks of ram would be bad.
If it still does not POST I would begin to swap each part one by one with other parts I have on hand.