Unplug your PC from the wall.
Remove HDMI cables, or whatever other display cables you're using.
Hold power button for 30 seconds, and press it a couple more times before plugging the PSU cable back in to the wall.
Connect the display cable(s) back to your PC.
Attempt to boot.
UPDATE: Why the negative vote? This is the first thing I recommend everyone to do when their PC suddenly doesn't boot correctly. It solves the problem 80% of the time. You need to completely discharge the system in order to know if one of your components has failed. Above are the steps on how to do it.