PC build won't start.

[king1]

only with dead hardware or the obvious monitor/connection issues

Power supply could do that...

One of the gotchas is plugging the video cable into the mainboards connector when you are using a graphics card, that could do what you are seeing.

Also check the input selector on the monitor, some of the older ones didn't switch inputs automatically reliably.

The other thing I would do is unplug everything not needed ie all drives, they are not needed to see something on the screen

ALso check the graphics card if you have one has a power connector and is connected, the CPU power connector is connected

and if its a ryzen note some of them don't come with onboard graphics and will require external card - but these should still work (on VGA at least) albeit badly... (and try with VGA if you're not already)

usually these situations are check the basics, then unplug the unnecessary and swap out components to identify the failed part...

HTH