I have bought a Seasonic S12II, and now after a few months it can't even go to BIOS. My motherboard lights up and also the fans spin everything works great, but it won't work. Also there are missing motherboard stands, is that a problem?
What do you mean "there are missing motherboard stands"? You mean the motherboard is touching the metal panel of the case? In that case there can be a short circuit preventing the system from POSTing.
You can try breadboarding. That is to take the motherboard outside the case. Put it on some non-conductive surface, CPU, RAM, graphics cards and the storage with the OS on it (Windows?) and see if boots up.