Bless your heart. There are several possibilities here. First, as you said, your plants may have frostburn. Since it didn't get down to freezing, you could probably have saved them by covering them that night. (live and learn)
But, there are some other things too. Did you harden your seedlings off by introducing them to the sun a couple of hours one day, bring them in, a couple more hours another day, bring them in, etc? If not, your plants may have gotten such a bad sunburn that they died.
Also, it's possible that your soil is still too cold for alot of tender annuals. If you put your bare hand on the soil for a few minutes, and the ground feels really really cold, that could be the trouble. Plants like sweet potato vine, tomatoes, zinnias, and other heat lovers will rot, die or at least be stunted in ground that is too cold.
I am sorry for your bad luck. This is when you're glad for garden centers.