You'll need to call customer service and hope they can track it down.
Numbers here Guide: Coronavirus Community Help and Support
Or advice from an O2 Guru regarding a way to get through:
If you call 202, 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.
Track usage, upgrade and get support with the My O2 appDownload My O2