Ok, have seen a fair number of instances recently where the OTA for Android 6.x (Marshmallow) has failed to complete fully and has left phones apparently with no OS and thus will not boot normally. Commonly all that happens is the phone will only boot/reboot to the stock recovery.

The good news is that in most cases getting back to a working phone will be possible.

The bad news is that if your bootloader is "Locked" then the methods available to get your phone working again will involve a complete wipe of your device.

You can take a very simple step to safeguard your precious files (photos, documents, music etc) by copying the whole of your internal storage on to a PC before proceeding to accept the OTA.

Edit: Of course doing this should figure in your normal backup routine anyway. You have been gently warned [emoji6]