I recommend two things.
First of all hold down the power and sleep button on the top until the screen goes black AND let go of both WHEN YOU SEE the apple logo.
That is a complete reset and if any of the network settings is amiss it will fix them... you could also get a paper clip and pop out the SIM and then push it back in.
If that still does not work - I would recommend you restore in iTunes and that will put the phone back to factory state.
Personally I have noticed that from time to time I lose signal myself and have to turn it off and then on to get it to work again!