Spring can be a very dangerous time for fruit tree blooms. If cold weather hits when the buds start to swell and bloom then some or all of the blossoms can be killed. Generally it takes temperatures below 28 degrees Fahrenheit to damage blooms, but a great deal of variables may come into play - such as how long the temperature stays below freezing.
Once the bloom is killed the it may take some time to recognize the damage. You may be able to see the damage the next day if temperatures rise significantly. The easiest way to tell if the bloom is still viable or not is to look for the pistil in the center of the flower. In the picture the petals,anthers, and calyx have been removed. If the pistil is nice and green (bloom on the right) then it survived the weather. If the pistil is turning brown or has a water soaked appearance (bloom on the left) then the bloom is dead and should drop off within a couple of weeks.
In a normal year, you can loose up to 90% of the blooms and still have a good crop of peaches. Bloom thinning due to cold weather can also save you some time later on in the season when it is time to thin some of the fruit off. Unfortunately, thinning due to cold is not very selective, which means you may have fruit on some twigs and none on others.