Top up failure: Unable to process the payment

[Jay41]

Hello!

I have been trying to top up my O2 account, but I keep receiving this message: 

  "We couldn’t process your payment

Sorry, it’s not been possible to take your payment. Your phone has not been topped up. Please try again or use another card. If the problem continues you’ll need to contact your bank. Try again."

This has never happened to me before, and I don't know what to do. I checked with my bank and there is nothing wrong. Please help! I don't want to lose this number. 

P.S. It would be helpful if I can have an offical email of O2, so I can directly email them.  

[MI5]

@Jay41 

No email, sorry.

All ways to top up here Guide How to Top Up (PAYG) 

or You'll need to call customer service.
Numbers here Guide: Coronavirus Community Help and Support 
When it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.