Make sure you didn't accidentally put your phone in airplane mode under settings.....I've done this before when I went to turn the WiFi off or on.....it's easy to hit the airplane mode switch by mistake.....
I had this same problem the last couple of weeks. I found out yesterday that AT&T was making the switch to 3G in my area. The same thing might be happening for you. It would make sense that they convert as many areas as possible before the 3G iPhone release.
aaargh! my beloved (jailbroken) 4-month-old 3G packed up yesterday - anyone got any ideas before I take it in for repairs?
first, I noticed it was only vibrating, not ringing, when someone called
then last night, immediately after I made a call, the display vanished too.
now - people can call me but the phone rings (so quiet, it's inaudible) and I cannot take the call anyway because the display is not there and no matter what I press, nothing happens
it's fully charged by the way and can still be viewed by iPhone browser so no data lost so far