You'll need to call customer service.
Numbers here Guide: Coronavirus Community Help and Support
When it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.
If you want the phone unlocking then O2 need to do it for you
Track usage, upgrade and get support with the My O2 appDownload My O2