|Well, you don't need a hard drive installed at all to get POST screens. So that eliminates your hard drive and OS as the problem.|
All you do need is CPU/HS/fan, graphic card/chip, 1 stick of RAM, monitor and keyboard. So, there are four suspect hardware items. PSU, CPU, motherboard, RAM. Of those, I would say that either the CPU works or it doesn't.
You could try reseating all the RAM, cards, cables, etc. to see if you just have a bad connection.
You say this started a month ago. How long did the rig run fine prior to that?
If the CPU fan turning at all when you try to start?