First off - welcome to TechIMO
My standard procedure when I run into that type of problem is to pull the MB out & build it outside the case (just lay it on the table on top of the antistatic bag).
Clear the CMOS as someone else suggested, then just plug in the CPU and fan, one stick of RAM, and a VID card - and see if it will POST.
If it doesn't POST - you have to swap out the RAM/processor/vid card with known good parts until you find the problem. (I know - that can be difficult if you don't have any spare parts laying around, but I don't know any alternatives.)
Once you get it to POST, put it back in the case and add one component at a time. Check after each component is installed to make sure it still works.