See if any of the advice in the guide below helps @Chelkay
Guide: How can I sort out my network issues?
Guide: Is the network down for me or everyone?
Try swapping sims with someone who's phone is working - that will show if the issue is your phone or sim card.
If you need a replacement sim, the quickest fix is often to visit your local store - take photo ID, and they will swap the sim straight away.
Otherwise you need to contact customer services and they will post one out to you - which will disconnect your current sim.
Guide: How to find help & contact O2
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 Do not publish personal details (email, phone number, bank account).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
