Firstly check your sim in another phone to see if it works...if it does its the phone - antenna issues. if the sim doesnt work in another phone its most likely a problem with the network so best try contacting them or another sim
if it does turn out to be the phone try to restore your phone as a new, if this makes no difference check to see if your in warrenty still and Apple will repair/replace your phone :0)
Hope this helps ..
Use the following steps to restore your iPhone.
***** IF YOU CANT SEE YOU IPHONE IN ITUNES OR IT IS STUCK AT THE APPLE
LOGO - POWER DOWN YOUR PHONE - HOLD DOWN THE HOME BUTTON FOR AROUND
6SECS WHILST CONNECTING TO ITUNES - RELEASE BUTTON - ITUNES WILL THEN APPEAR WITH MESSAGE
SAYING IT HAS DETECTED IPHONE IN RECOVERY MODE & CONTINUE TO RESTORE
YOUR PHONE - FOLLOW FROM STEP 6! ) *****
- Make sure you have the latest version of iTunes installed and running before attempting to update.
- Connect your device to your computer.
- Select your iPhone when it appears in iTunes under Devices.
- Select the Summary tab.
- Select the Restore option.
prompted to back up your settings before restoring, select the Back Up
option (as seen in the image below). If you have just backed up the
device, it is not necessary to create another.
the Restore option when iTunes prompts you (as long as you've backed up
your device, you should not have to worry about restoring your phone).
- Once the restore process has completed, the device restarts and displays the Apple logo while starting up:
For the iPhone only, after a restore, the iPhone displays the "Connect
to iTunes" screen. Keep your device connected until the "Connect to
iTunes" screen goes away or you see "iPhone is activated." If iTunes
does not have an Internet connection, you cannot complete this step.
final step is to restore your device from a previous backup. Once your
device is restored and-in the case of the iPhone-in the process of
activating, you should be able to see the following in iTunes:
Select the backup you want for your device and select the Continue button to complete your device's restoration.
If you do not want to restore from backup, select "Set up as new...