Tasteless coke

Hi there. I have purchased a 2l coke 2days ago from a local store in our area. However when I opened the bottle and poured myself a glass to drink, after a few minutes of purchasing it, it was flat. Immediately I asked my daughter to try it, she too complained that it wasn't normal. I then took it back to the shop but the lady there, told me that they cannot take it back or give me a refund. I was extremely disappointed as this has never happened before. I paid R22. 00 for the bottle and the lady refused to refund or even try the coke herself as it had no gas. I'm highly upset. And would like to be refunded or anything if possible.