@AlinJ
See if you can access the web version https://mymobile.o2.co.uk/
If so, try deleting and reinstalling the app or ask O2 to reset your account.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.