It didn't pollinate. The reasons for that happening are various. Plant not having enough energy, this or that deficiency, not enough light, extreme temperatures, etc.

Agree but there is one other possible cause too - thrips. They will sometimes snip blooms off. Do a close up exam for any insects to rule them out. If no signs of them then it is a condition called "blossom drop" that you can Google for all the details about the various causes. There is also a FAQ here about it.

Dave

This time of year, extreme temperatures can be a main reason for tomatoes not pollinating. Where are you? When were your tomatoes planted out? If you're shooting for a fall crop, and it hasn't cooled off in your area, that's exactly what you should see.