The three important rules to turning this around are: DON'T IGNORE THE REVIEW DON'T DELETE THE REVIEW TAKE A STEP BACK BEFORE RESPONDING
Often the worst thing you can do is either delete the comment or ignore it. This is the digital age and someone somewhere will see the comment and if you delete the comment it's likely to end blowing up in your face.
What you should do is:
Start of by apologizing to the customer on the same medium they made the complaint asking for more details if necessary along with:
It's obvious that you want to resolve the issue amicably, resend the product, offer a refund, offer a replacement etc. Do what's needed to fix it. When it's resolved go back to the review online and post another comment to say it's been fixed. This encourages them to say thanks and this then turns a negative review into a positive review!
We live in the real world and we know that this strategy will not always work, sometimes customers just want to vent so what do we do then? Pretty much the same as we have done, be reasonable offer to fix the problem or make it better. You can always detail your offers on social media for the world to see, and sometimes others will come to your aid and tell the person to pull their head in.
On the other hand sometimes bad reviews are vexatious false or plain libelous, in these sorts of cases I suggest engaging the services of a lawyer and getting professional advice.