If you turn your device off and wait 5 minutes ,then turn your device on again,as this can help in most cases.
If your problem hasn't been sorted then
check the signal in your area by visiting this link http://bit.ly/ggservice
If it is still not fixed you can now do a maual roam:
- Go to your phones "settings".
- Select mobile network options.
- Change the Network Selection Mode from Automatic to Manual
A list of available networks will be displayed after a few moments.
- Select any other network that is not your home provider network. After a few moments you will see "No Service" on the display.
- Now select your home provider(which is o2) network and change the selection back to Automatic.
Hope it all goes well