Check the service status here for any maintenance or issues
http://status.o2.co.uk/ or pop into any o2 shop and try a new sim card.
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.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)