Hi there and thanks for visiting the community.
If you could please try turning your device OFF,wait 5 minutes,then turn your device ON again,as this can help in most cases.If your issue has not been resolved,check the signal in your area,by visiting or clicking onto the link below.
Please be aware that the website is updated hourly, so there may be a current issue that is not showing at the moment. Keep checking the signal checker website over the next few hours or so,cheers 😀 .http://bit.ly/ggservice
If the signal checker is showing NO issues,you could also perform,a manual roam,which basically is searching for networks on your device.Go to your phones "settings" and Select mobile network options and Change the Network Selection Mode from Automatic to Manual and list of available networks will be displayed after a few moments.Select any other network that is not your home provider network and 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 😀 .