I have had my site for 3 years and all of a sudden my discount codes are not working. When someone uses a code the site says "congratulations you have redeemed your voucher" but then it doesnt let them go any further (ie Step 3) to complete payment.

I havent done anything and I know I need to update as I'm on an old version but surely it shouldnt just SUDDENLY stop the codes working?

Can anyone please help?

Ali