I've got a friend who's with Orange and regularly gets a massive phone bill each month (3 phones, laptop, dongle and maybe other things as well on there). The bill is usually £200+ and she regularly gets cut off for not paying on time. When she has the money, she rings up and pays the bill and then gets re-connected, so I wouldn' think O2 would be any different. I can't see you having to start a new contract, unless that's what you want. My friend just gets re-connected on her existing number/contract.