No restaurant runs perfectly every day.
When the Michelin people are out looking for restaurants to award their famous stars to, they visit each place several times, not just once.
Anyone can have a bad day.
Think about the small details of your restaurant visit. Was the server attentive, but not awkwardly present? Was the menu thoughtfully put together. Was the food carefully plated? Was the food as good as it could have been?
Not every restaurant deserves Michelin stars, but that doesn’t mean you shouldn’t enjoy a visit there. Don’t be a snob, don’t compare a fried chicken place (which I love), to a classy fine dining restaurant (which I love).
Remember, just because a restaurant used to be great, doesn’t mean it will always be great. Pay attention to the details, poke around a bit, and have some fun while you are doing it.