The first thing I would do is to unplug all peripherals from the MB including the HDD and GPU. Plug the monitor into the MB graphics port. Replace all RAM with a single stick of known good RAM & rotate among all available slots. If it powers up in that state the problem is in something you unplugged. If not, replace the PSU with a known good one. If none of that works the problem is in the MB or CPU. You would need to replace each one at a time to isolate.