Really wanna know?
Yes, that's right. Do nothing.
If you must vent, do so in the privacy of your own home. Cry, scream, throw something. Give yourself one day to wallow in self-pity. Eat chocolate or indulge in a marathon of guilty pleasures. Pet your cat, dog, snake, or hamster. Then, pick yourself up, throw back your shoulders, and toddle over to the keyboard. You've got more important things to do.
What you shouldn't do is react in public.
Do not engage the reviewer, whether or not you agree with their assessment. Do not have a bunch of your friends respond on your behalf. Do not whine on social media about how unfair some reviewers can be.
If, upon self-reflection, you realize that maybe the critic is right about a particular weakness in your story, fix it if you can. If you can't fix it in this book, learn from your mistake and make your next book better.
If the criticism has no merit, why would you spend a moment considering it? Let it go and move on. You have other things to worry about--plot lines, deadlines, cover art, edits...
Why on earth would you waste time on a stranger's opinion? When reading your reviews, act like a reader, not a writer. If the blurb doesn't appeal to you, choose something else to read. You are responsible for your own happiness. Don't let a faceless person with a name like "LuvMeKindleFreebeez" on Amazon determine your worth.
You know what you are. Write on!
For tips on writing and fun articles, visit Gina's Articles For Writers page: http://www.ginaardito.com/ArticlesforWriters.html
Need editing services for your manuscript? Gina is proud to announce the launch of Excellence in Editing: http://excellenceediting.com/