Can you log in online https://mymobile.o2.co.uk/ ?
If so, try deleting the app, rebooting your phone and then reinstall the app.
You can reach the Payment Management team on 0800 902 0217.
The opening times are:
Monday to Friday – 8am to 8pm
Saturday – 8am to 6pm
Sunday – 9am to 6pm
All ways to pay here. A direct debit is favoured Guide: How to Pay Your Bill (Contract)