I just went through this with my M9 and it looks like the linked Youtube video is down. :/
[For this, I'm assuming the phone is S-On and the bootloader is locked. It also assumes that the eMMC chip hasn't hit it's End-of-Life and died]
The answer starts off with getting the right aosd.img for the phone. Then, with the phone in fastboot mode and connected to the PC:
Type "fastboot flash aosd aosd.img" and press Enter. This will give you back Download Mode. From there, the choices are:
If the bootloader is OK, you can boot the phone into Download Mode and try to re-flash the firmware using the official RUU .exe. [If the bootloader isn't OK, the phone may go into a reboot loop: To break the loop, hold the Power and Volume Up buttons]
If you are unsure of the bootloader, you can try reflashing the phone by putting the RUU .zip file onto a micro SD card and flashing from Download Mode.
The worst case scenarios I've seen so far are:
If the phone cannot be reloaded via the RUU and you're stuck S-On, you can use an XTC2Clip to go S-Off and reflash the firmware.
The eMMC has exceeded it's life cycle and gone completely corrupt and unrecoverable. At that point, the mainboard would need to be replaced or the original board would need to be reworked to put on a new eMMC chip (iirc, a new chip is ~$5. The main cost is having a reball setup and proper jig).