Common problem.
Use https://accounts.o2.co.uk/signin?
If that works then delete the app and then reboot the phone and reinstal
If all fails
You will need to contact O2.
Message them on Social Media. Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG)
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
