I have been cut off and can't even ring O2 to sort it out. Apparently my bank cancelled my direct debit and they don't know what happened! I then got a text from O2 that said my last bill that I missed and my most recent one will be both due on 25th of jan so I though I could pay it in full on the 25th!. But this morning I am cut off.
Because the direct debit was turned off you'll need to pay manually. You can pay online using a laser/credit card, or in a Post Office for cash etc. I've resumed the account so just restart your phone but if you're going to be late with the payments, make sure to let us know so we can intervene with the system on your behalf.