Yup, power supply should be the first thing you should check. Get a spare power supply and test with that. Just unplug the existing power connectors, and hook up the leads from the spare. No need to install it physically.
Unplug all of your drives, internal and external, until you can get a good POST. Unplug everything else external except for the keyboard and monitor.
If you have any add-in cards (PCI or PCIe) temporarily pull them out. If you don't have integrated graphics, you might have to get a spare GPU to test with.
Test your RAM one stick at a time.
If you do all this, and still don't get a POST, the motherboard is probably dead. Check your CPU socket and make sure everything is still seated. Maybe pull the CPU and reseat it with new thermal paste.
No need to worry about the hard drive or OS until you get this far ...
