HTC One M8 - Locked after Android 6 update
I have recently discovered the same problem. Here are the steps which helped in my case (un-rooted phone just stopped loading after vacations):


I plugged in the cable to PC-USB. Th phone was not recognized.
I pressed on the phone something like "checksmartsd"
The phone was recognized by the PC and started as usual.


By "reboot" everything was fine. By "shut down" and "launch" the menu appeared again (phone not plugged to the pc). I chose the same "checksmartsd" and it worked.

I have no idea why it worked. I hope it helps somebody.

Best,
Eugene