As much information as you can give would help. Is that a completely black screen, or for example black with some red triangles one it? What was it doing when it said "update error" (i.e. what was it trying to update at the time)?

Try booting while pressing the "back" key. If that takes you to a white screen with the word "fastboot" near the top, plug the phone into the computer. If this changes to "fastboot usb" then it's possible for you to attempt to run the update again. If you can't get to the fastboot screen then I suspect that your only option is to take it to HTC for repair. HTC offer a 2 year warranty, so you are probably still in cover, and if their update breaks it they will fix it.

 

Thanks. I really meant "what was it updating"? The update replaces many elements, e.g. bootloader, recovery, ROM, radio, and failures at different points are easier or harder to fix (ROM is straightforward, radio or bootloader almost certainly require return to HTC).

So I take it that the screen stays off, keys don't light up, and you cannot get into fastboot mode? Does the charge light come on if you plug it in, or is it completely unresponsive? No charge light would be a very bad sign.

Have you tried pulling the battery out for a minute, then seeing whether it will boot?

If not, and the screen remains black, then the only thing I can think of would be to boot while pressing the "back" key down (after another battery pull, & hold it down for a little while to be sure if the screen is off), connect to the PC and see whether the update program can see the phone. If it's just the screen driver that's not working that might work. But to be honest this is a long shot - I think that HTC are probably your best bet.

 

Afraid not. The radio firmware actually controls a lot of the hardware, and if that gets corrupted it's beyond the user's ability to fix. As for the bootloader, you need that to be working in order to flash the device. So if either of these is corrupted then you need specialist kit, not just a PC and a USB lead, to do anything with it.

If booting was erratic there may have been some other issue to start with. At this point I think the only advice I can offer is to contact HTC. As the upgrade is their official software they should fix it for you - they did for another member who had a problem when running it last year.