Boot loop after updating HTC One M8 to Android 6.0 Marshmallow

Couple of days back, I got the notification to download and install official Android 6.0 (build 6.12.401) in my HTC One M8 (not rooted, was running Android 5.0.2). As I downloaded and installed it, from that point it's gone to boot-loop state and showing only HTC logo. Now I have tried to do Factory reset that ends in the same loop, recovery mode which does nothing and stays on that screen with exclamation sign, installed ADB on a friend's advice, *but the mobile's debugging is not on*. Any advice or explanation of the issue and the fix?