Solved the same problem here
I just had the same problem and got it working with a fast-boot followed by a factory reset. As I didn't know exactly when it was turned off, I could only be sure about when it was on (after pressing power button and having the boot animation loading). Based on that, I pressed the power button for about 6 seconds to turn it off.
Having it turned off, I followed these steps:
1) Power + 'Volume -' for 3 seconds
2) Release Power, but keep 'Volume -' pressed until Fast-Boot appears
3) Search for (navigate to) "Recovery" by using only 'Volume -'
4) Confirm 'Recovery' with 'Volume +'
5) When the android robot appears (probably with a 'no command' message), hold 'Volume +' and, after that, tap Power
6) Go to wipe/data factory reset and confirm it with Power
7) Go to 'Yes', confirm it with Power and wait. For some moments it may seem dead, but it will be doing its job
8) After it is erased, go to 'Reboot system now' and select it with Power
9) Wait until it is restored by itself
I hope this helps anyone with the same problem