Firstly check the signal in your area to check for any faults with the network in your area. You can do so by visiting this website www.bit.ly/ggservicestatus
If everything is alright with the signal in your area then you should try a manual roam which will update your connection with the network. Click on this link to view how to do a manual roam http://is.gd/manualroam
You should also try turning flight mode /airplane mode on and then off again.
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