Answer: It sounds like the plants may have suffered from frost, since your average last frost date is May 10th, especially if they are in a frost pocket (a low-lying area that collects cold air as it settles). More likely though, the plants are suffering from transplant shock. This occurs when roots are disturbed and damaged during transplant, especially if the plants were rootbound in their pots. Were the plants hardened off? The hardening off process is a gradual acclimation of plant to the outdoors after they've been pampered in an indoor environment. They aren't used to full exposure to the elements. The process can take 5-10 days, depending on how the plants adjust. Start by leaving plants out for a few hours in a place sheltered from direct sun and wind. Leave them out a little longer the next day, and gradually expose them to more sun and wind. Make sure the soil doesn't dry out.
At this point, mulch around your plants to help soil retain moisture. Tomatoes are survivers -- even if the top of the plants die, often buds lower on the stalk will take over. Best of luck!
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