A great tip from The Apple Blog!
To get my signal back I used to turn the phone completely off, and boot her back up. This solution was alright, but it took a lot of time and it occasionally took several attempts for my phone to actually find the network. So, I have new method. I simply:
Turning Airplane mode on disables the wireless features of the iPhone, and then turning it back makes the phone re-register on the network. I’ve had better success getting my signal back by toggling Airplane mode rather than rebooting the phone. Good luck!
(Via The Apple Blog.)