I asked this question in our Answers beta, and I got these responses for you. Let us know if any of them fix your problem!
From tlcgraham:
Sounds like a loose connection for the antenna.
From wtylko:
Every time you loose service a much faster way to get service is to turn AirPlane mode on and off. Also have you updated your iphone to the latest software? If you haven't you should do it. And if all this does not work try restoring your iPhone an and get it swapped out at the genius bar. But don't tell them you dropped it. They have no way to find out! Also un-jail break you're phone before you take it there.
From bmalam:
Hate to state the obvious, but have you tried a reboot?
[quote|Kyle Wiens]I asked this question in our [link|/Beta|Answers beta], and I got these responses for you. Let us know if any of them fix your problem!
From tlcgraham:
Sounds like a loose connection for the antenna.
From wtylko:
Every time you loose service a much faster way to get service is to turn AirPlane mode on and off. Also have you updated your iphone to the latest software? If you haven't you should do it. And if all this does not work try restoring your iPhone an and get it swapped out at the genius bar. But don't tell them you dropped it. They have no way to find out! Also un-jail break you're phone before you take it there.
From bmalam:
Hate to state the obvious, but have you tried a reboot?[/quote]