Back to basics, reset cmos
Disconnect everything that you don't need to get a post, that includes hdds, dvds, memory, gpu.
Yes I did say memory and gpu, as they will give you error beeps and then you know board and cpu is ok.
Add 1 stick of ram, check for error beeps and then add gpu, and if all ok you should get a display.
Now its a case of adding extra ram and devices one at a time until you hit a problem or it magically goes away.