Certified GKH Gardening Expert
There are two main possibilities: lack of fertilization or blossom end rot.
With the scarcity of bees these days, it is possible that your squash flowers are not getting fertilized efficiently. In this case the tiny unfertilized squash will stop growing and eventually rot. To fix this, you can fertilize by hand or plant bee-attracting flowers nearby.
The other possibility is blossom end rot, a disorder caused by deficiency of calcium in the growing fruit. Please see this article for advice: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm