I can't use the power button on my Lumia 520 for anything. It's dead after the Lumia Black update.
How can I solve this?
This is probably a bug . If your Lumia 520 does not wake up, you can force a reboot or soft reset or hard reset. This is possible by holding down the Volume Down and Power buttons together for about 15 seconds or until you feel the phone vibrate. Also you can report it to windows phone developers.