Check the signal in your area to check for any faults with the network in your area. Do so by going to this link www.bit.ly/ggservicestatus
If everything is all right with the signal in your area then try a manual roam which should update your connection with the network.
Make sure you have the correct apn settings for your handset to do this: Text "Settings" to 2020. If this doesnt work, then use one of the link below based on the handset