Have you tried your sim in another phone to check whether its a sim issue?
Have you checked your network status? https://status.o2.co.uk/
If all ok then I'm afraid it is going to be a case of calling O2.
All numbers in this guide Guide: Coronavirus Community Help and Support
Use any or all. Press any option to get through
*The Game Is On*
Track usage, upgrade and get support with the My O2 appDownload My O2