The following assumes that you live in England or Wales (and that you're not disputing the debt):
If you've not had any contact with the bank for over 6 years (including, for example, you asking them for a statement of the balance), you've not made any payments to them within that period, and the bank has never obtained a CCJ against you (in respect of the debt), its likely that the provisions of the Limitations Act 1980 will apply.
If so, the bank (or the debt collection agency) can no longer pursue the debt through the courts. However, you still owe the money and you can still be chased for it. In particular, the bank (or debt collection agency) can register the debt with credit reference agencies, effectively barring you from obtaining any form of credit until you repay the money.
Chris