ammm. that thing also happened to my M2 when i flash kitkat. (i downgraded my phone lollipop-kitkat, for camera).
NOTE: my phone is BOOTLOADER UNLOCKED, so i can flash firmwares, root it, etc.
the first thing i did is:
reboot the phone - hold power button and volume up. when it vibrate 1 time, let go.
^if that doesnt work (it doesnt work for me.) do this:
hard reset (like taking out battery) - hold power button and volume up, just keep on hold until it vibrate 3 times (about 10 sec i guess.)
then next thing to do is to flash the firmware... i downloaded the firmware using Xperifirm and flash it using Flashtool.
