Just as Discobay said,

Its most prob the BIOS. Clear it, I had this happen to me few times after overclocking the CPU too much that the mainboard shuts off to preserve the motherboard. Clear the CMOS and then see what happens. 650w PSU is sufficient unless you have more than 1x high-end graphics card.

If that doesnt work, unplug the main power from the PSU, wait like 5mins, plug it bak in, clear the BIOS, then try powering it up. Should work.