htc one m8 phone won't work after downloading latest upgrade

[AC Question]

Downloaded latest system upgrade, now I'm getting red triangle symbol, phone tries to reboot continuously but won't start up.

 

[Rukbat]

Bring the phone to your carr They sent you an update, they broke your phone, now they get to fix or replace your phone. (It's their liability.) You may have to end up calling tech support, then going up several supervisory levels until you get someone who can understand that a liability issue has nothing to do with a warranty, except the implied warranty that if they break your phone, they're responsible for the damage. When you get someone who says, "well, we can replace the phone ..." you got the right person. Accept the offer. (They can't replace your data - it's your responsibility to keep it backed up.)