M7 locked, unrooted, stuck in bootloader menu

[m7misery]

My Dad's HTC One M7 is stuck in the bootloader screen and is unable to boot to the main OS.

The phone has never been unlocked or rooted, I've never even seen the bootloader screen until now. The first thing it says in the bootloader screen is ****LOCKED****.

The only information I know is that prior to this, he was prompted for a system update which (he says!) looked official.

I've tried every option in the boot menu (fastboot, bootloader, recovery, recovery reset) and it just goes back to the main screen.

Is there a key combination that I'm missing? Was the system upgrade corrupt? I know next to nothing about Android bootloaders, so any help would be appreciated. Thanks.

Edit: Before anybody says, I have tried Googling for hours but all the results are from people trying to load a custom ROM or unlocking the bootloader - nothing about a 'normal' installation.

 

[Forsh]

I have this exact same problem.

Went to update my phone yesterday, left it running and when I came back to it, the screen was 'on', but displaying a dark navy/black colour i.e. you can tell the screen is on, but there's nothing there.

I can access the bootloader (hold down power button and vol. down), but nothing from this menu will do anything. Have tried fastboot, recovery, factory reset etc.. nothing will. Sometimes the HTC will vibrate and the white HTC screen will show (as if it's turning on normally) but then phone returns to the black/navy screen.
Sometimes, the phone will show a red triangle and exclamation point after attempting reboot/recovery/reset.
Sometimes, the phone will show battery charging symbol (only time the amber LED charging light shows up) and then jump to green 100% symbol (with no LED). Holding down the power button then shows white HTC start screen and then the dreaded black/navy screen once again.

Also to note that this bootloader is also 'locked' and 'unrooted', previously working perfectly fine and this problem only came about from a so-called update.

 

[MrDroidi]

You need to use the RevOne tool on Windows and RUU update it back to Factory

 

[MrDroidi]

Your update probably failed and you soft bricked it

 

[MrDroidi]

You can also take it to your carrier repair store and they will do that for you usually