Depending on the types of plants you had and how cold it was, your plants may recover. Some annuals, such as pansies and snapdragons, will grow back after a light frost. The best you can do is keep them watered and see if they grow again. Within a few days, the extent of dieback should be evident. Trim out the dead portions of the plants. Often new shoots will sprout from beneath the damaged areas, similar to when growing tips are removed in pinching for bushier growth. Use this as a lesson to avoid buying plants too early in the season. If you do get plants a bit early, protect them from frost by covering them at night with a sheet, rug, or tarp. Or bring them indoors on frosty nights.