From what I can tell the power button does 4 things. 1) turn the phone on. 2) turn the phone off. 3) take a screenshot 4) power cycle the phone. The only thing that doesn't work is turning off the phone. If I have to I can plug the charger in to turn the phone on, but admittedly if it gets to that point buying a new phone is a priority.
A little history. I bought this phone from Amazon in 7/12. Within 2 weeks I couldn't turn the phone off. Contacted HTC, they said to send the phone to them and they'd send it back in 6-8 weeks. There was no option to expedite this. As I live alone and don't have a land line this meant no phone for 6-8 weeks. Unacceptable.
Perusing the googles I discovered my power button problem is common with HTC phones. Someone said to spray the switch with silicon lubricant, as my option was that or buying a new phone I tried it, and it worked. The button has been intermittent for 2 years, it finally failed completely last week. Spraying with lubricant no longer works, and google says the problem is much worse than it was 2 years ago.
Phone is backed up, don't care about contacts/messages, and there are no passwords on the phone. I tell everyone I know to avoid HTC phones, this phone has been crap from the second week I bought it.