You also don't mention what sort of PSU you're using unless I missed that, please tell us - both brand and wattage. I've run into problems before where a PSU and mobo are incompatible due to the PSU not quite delivering the tolerances required by the mobo on the ATX12V CPU line, that was years ago on a P4 build, but it's possible issues like that still exist today.
The other really stupid thing I've seen before is a defective case with the reset switch broke in the pressed position. Either way do what Nacelle said, run the system outside the case with minimal RAM, no drives, & if you can find a PCI video card, use that, & short the pins normally used by the power switch with a screwdriver. If I have to do that, I generally take the mobo box & set the antistatic bag the mobo was wrapped in on top, then build the system on top of that, putting the PSU near the box. Keep the PSU plugged in while working and if you have to move a lot, touch the chassis of the PSU to release any static energy buildup.
If none of that gets it, you've got something defective. My first thoughts would be to reseat the CPU, making sure no pins (if applicable, I haven't worked with AMD in a while) are bent. Clean the heatsink & CPU off until they are pristine using any of the 100 easily google-able ways of doing so, apply a BB size drop of thermal compound to the center of the CPU or HSF, attach both and try again. Plug your HSF back in at this point and give it a go. And if it's still not working at this point you may find it simpler to drop some cash at a local shop for a diagnostic if you absolutely need it running before the weekend. I know Stone Computer in Ft Wayne charged $45 for that a few years back. Good luck!
