Had a Galaxy S2 that would never work on after it updated to 4.0. It was some sort of inherent issue with the firmware that was fixed with the official release of 4.1. I also had a Droid Razr that wouldn't work on. It would just freeze on a black screen. You could literally feel the heat building up and you'd have to wait for it to drain.you just hold the power button and it restarts the phone.... same as if you removed the battery
For a second just go google "hard reset not working android" and you'll see the numerous issues people have had. Nothing beats being able to yank the power source at any time.