I checked everything and cellular data is on and under general then tap about... It says that my network is not available
Hold the Power and Home buttons together (Home is the button on the front, with the square), for about 40 seconds, until you see the white Apple logo.Then let go, and allow the device a minute to reboot. It will then function as usual.
Let's try the more aggressive approach (which will delete all settings from the phone).Settings--General--Reset--Reset all Settings.If this doesn't help, it means the SIM card went bad - you need to pick up a new one at any local ATT store (they won't charge you for it). It is a known issue with some batches of iPhone 4s SIMs, and the only way to solve it is to replace the SIM.