Are you a pay monthly or pay as you go customer @Shazza13 ?
First thing for both would be turn your phone off, take your sim out and clean it with a dry, lint free cloth. Carefully put it back in your phone and turn it on.
If that doesn't work, see if either of these guides helps:
Guide: How can I sort out my network issues?
Guide: Is the network down for me or everyone?
Finally, try getting in touch with customer services - you can use Skype to call the 0800 numbers, or via social media:
Guide: How to find help & contact O2
Guide: A Guide to Skype
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
