If the blooms die, there will be no tomatoes. Normally, this is a problem when one has very high daytime and nighttime temperatures. Reduced production starts when the daytime high is above 85°F and the nighttime low is above 72°F. "Heat set" tomato varieties are more heat tolerant.
Other causes: using too much nitrogen, pests and some fungal problems (Botrytis; bacterial spot). Ironically, too many blooms can also be problematic.