Discovery card error but nothing they can do

I went and did my weekly shopping on Friday, paid with my Discovery Card, only to be informed that it was not going through as there was an error message. After being treated by the store like a ********, I was marched to the office for the supervisor to call the bank. After many calls, 45 more minutes of my time and cryptic messages reinforcing my ********ity - I was informed that there had been some ***** at FNB, my card had been erroneously frozen and due to a technical error could not be restored until the end of the day. Discovery were clear that nothing could be done to enable me to take my groceries. I eventually returned home extremely frustrated, phoned Discovery Card again and eventually spoke to a supervisor. Same story but she said she would escalate the problem. She called me back about an hour later to inform me that the error had been rectified and the card restored. She apologised and told me the problem had been beyond their control so aside from an apology there was nothing she could do. This is unacceptable, not to mention unprofessional. I will be cancelling my card - credit card services need to be handled very carefully and it is clear Discovery don't.