If your phone is technically on but freezing, and the soft reset doesn't work, you could try the instructions here if you're familiar with Navifirm and Nokia Care Suite
[GUIDE] How to flash a dead Lumia 800 RM-801 - xda-developers
Basically it details a Nokia firmware recovery procedure if the firmware on your phone screws up. You would need to ensure you download the exact same firmware as your phone (Nokia Care Suite can give you the serial code, which you can then look up on Navirfirm).
This procedure helped when I did a firmware update on my phone which had worked previously, but which for some reason decided to make my phone cycle on and off without allowing me to use it. I copied my firmware files to a RM-817 folder which enabled Nokia Care Suite to perform a firmware recovery that it otherwise wouldn't have been able to do :)