I've had this happen a few times over the years.
It appears the PC is dead and nothing seems to fix it, even a BIOS reset.
What did work was pulling the plug and leaving it unplugged for 5 min.
I unplug the cable, then press the PC power button (not saying you need to, just what I do), wait 5 mins, plug power cable back in, turn it on and it works.
I have a Gigabyte X58 DS4 motherboard, it happens around once every 3 - 6 months, haven't a clue why, now if it does, I don't even bother resetting the BIOS, I just unplug the cable as described above. When I plug it back on and power on the BIOS has reset to default values automatically, hence I need to enter date and time, set disks to correct mode, set up any other stuff I need.
Hope that helps
I had this problem with a Gigabyte x58a mobo. It was the PSU...