Run the minimum 'set' of memory just to see if it POSTS or boots..

I had something similar and removed a set of DIMM's at the time from Bank 1 leaving the others in Bank 0 alone. It booted up less 8g of RAM but I then swapped out those DIMM's in Bank 0 with those in Bank 1, knowing those 2 DIMM's worked and it did not boot.

I had 1 DIMM out of the pair in that Bank that went bad. Replaced those pair and haven't had an issue since and RMA'ed the bad pair.

I build all my stuff so not sure your level of 'get in there and mess' you might have but I would rule out anything and everything including resetting BIOS at the jumper level and removing the cord from the wall for 10+ mins. I also had a situation where the power supply triggered a limit-relay on a brown out [much like what you were saying] and removing the wall cord allowed the system to "discharge" enough as well the PSU to "reset" that plugging it in worked first time.

Try everything.