My HTC One is showing, on a black screen,

[AC Question]

a picture of the phone's shape with an exclamation mark inside, then it flashes to the HTC white startup screen, then back to the exclamation point. Any ideas on what to do? It started doing this last night after it asked me to run an update (which it has never done before and I've had the phone for nearly 2 years). Thanks for any help you can give.