Check the Sims tray for any dust or lint.
If you can try another Sim card in the phone to rule out the phone at fault or try your Sim in an alternative phone. Process of elimination.
Give the Sim card a clean.
Try visiting the O2 High-Street Shop and ask them for a blank Sim for your contract type, ie ; Payg or Contract, and ask them to perform the Sims swap for you.
It can take up to 24 hours for your O2 Network signal to settle.
Turning the phone off and on helps to reset the connection.
Let us know how you get on.
Track usage, upgrade and get support with the My O2 appDownload My O2