Well let's take it one step at a time.
First, I would try to boot to recovery and flash a stable backup - you have one of these I hope? So power off the device, and hold vol up + power to turn it on. Should boot to recovery.
If you can't make it into recovery for some reason, you should be able to make it into fastboot/bootloader mode and use the FXZ thread to restore your phone to stock. To do this hold power + camera, should boot to a selection menu.
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