I would take out your motherboard from the case and attempt boot with only - PSU/GPU/Motherboard/CPU/Ram/CPU cooler. No need to connect up other components as of this test goes. Before booting, take CPU out and put it back in, making sure no thermal paste as dripped onto or into the socket. Re-apply thermal paste and put cooler back on. Attempt boot. If still red light the board may have been DOA, and needs to be replaced.