It would help if you put your location and/or hardiness zone in your profile. How much rain have you been having? Is this happening to all of your squash (including other varieties if you have them)? Are the leaves and vines in good shape? Some of my zucchini are showing the same symptoms, but it's blossom end rot because of the amount of rain we've had over the past 5 weeks. It's raining again as I write this.
Dan - "Welcome back my friends to the show that never ends..." Emerson, Lake and Palmer - Karnevil #9
Cut it in two lengthwise, stem too and see if there is a worm in it. If not, could just be conditions. Give it some time, they should start to develop.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/- Plant a Garden
I agree, what it looks like to me is a squash from a flower that didn't get (sufficiently) pollinated (it takes more than one visit).
The female flower comes with an embryo squash behind it. Even if the flower is not pollinated, the squash keeps growing for awhile, but eventually shrivels up and dies.
Have you seen any honeybees around your squash blossoms? Many of us are noticing the lack of honeybees this year. I had been hearing about the decline in honeybees for some years, but this year is the first I have really noticed it in my own garden. "Beekeepers across the United States lost more than 40 percent of their honey bee colonies during the year spanning April 2014 to April 2015, according to the latest results of an annual nationwide survey." http://www.sciencedaily.com/releases/20 ... 093605.htm
If you don't have sufficient pollinators, you may need to hand pollinate them.