Bad ram
Bad MB
Bad PSU
Bad CPU
Bad GPU
Bad install
Bad cables
Bad BIOS
The reasons are legion. There is an old computer troubleshooting technique called "eliminate and consolidate" which refers to starting with a minimal install of the CPU, one stick of ram and a GPU outside the box. Once you get it to post, you add one component or installation step and boot to test the setup. Wash, rinse, repeat. At the basic minimal install level, you can only replace until you find the culprit.