So this happened to me once before. I was traveling and quite suddenly in Boulder, CO. this started happening. This was about two months ago. I spent hours getting through the technical support machinations over at Cingular before they decided that I needed to replace my phone. So I have it shipped to my office in CA since I was due to fly back shortly. When I got into the airport and turned my phone on - voila! It was working again.... grrrr.
Fast forward to this week and here I am in Boulder again, and the phone is DOA yet again. At one point, my phone was working, making calls, sending and receiving email, and then later it wasn't. Nothing changed - that I could see. This is too much of a coincidence for comfort. Tonight, I did some research and discovered that this was a recent topic on SmartPhone Thoughts. I chucked to myself - yeah, I read this last time. Fortunately, this time, there was an answer further down the thread. On or around late April, new firmware was released:
http://www.htcamerica.net/support/software_downloads.htm
Worked like a charm. In about 15 minutes, I was able to sync my phone, make calls and receive text messages.
Be sure you back up anything important because this will flash your device.