Maybe there was a specific AT&T issue yesterday (I didn't experience any problem), but I have this issue from time to time in general. The car won't have any connectivity for 5-10 minutes while driving in areas that I've had strong signal before and where my AT&T iPhone currently has a signal.
In the past, I've tried soft reboots and hard reboots. Sometimes with success, but who knows if it's coincidental.
Kinda frustrating, but the only thing I do is submit a bug report after I get signal back.