Text "active" to 2020. Save and apply the settings that get sent back and then restart your phone.
Make sure you have mobile data enabled and that you can access the internet OK. If PAYG make sure you have actual credit on your phone.
If EVERYTHING is still failing, call CS to get your account checked out on 202 (contract) or 4445 (PAYG).
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.