Some things to check:
1 - have you got the right APN settings? See the link below to check:
https://theroams.co.uk/providers/o2/guides/internet-settings/2 - Is your sim working?
Try your sim in another phone (if possible) to see if that works.
Try another sim in your phone to see if that works.
If your sim works in the other phone, your phone is the problem.
If the other sim works in your phone, your sim is the problem.
3 - No access to another sim/phone? Try calling customer services to ask for a replacement sim.
4 - Pop into an o2 shop to ask for help.
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 Do not publish personal details (email, phone number, bank account).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
