What part of MI5's advice are you referring to @Gamer450 ?
The question was can you log in to the browser version ( https://mymobile.o2.co.uk/ ).
If yes, then try deleting the app, rebooting your phone and the installing it again.
If you can't log in online, or you can but doing that doesn't work then you will need to get in touch with o2 customer services.
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 Do not publish personal details (email, phone number, bank account).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
