You'll need to call customer service. 4445 or try the numbers here Guide: Coronavirus Community Help and Support
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
Try at 8.00am for th best chance of getting through.
When available, live chat will show here http://www.o2.co.uk/contactus
or try hanging around the shop pages.
The store would only be able to try and call customer service to as they wouldn't be able to add credit.
Track usage, upgrade and get support with the My O2 appDownload My O2