You did not send a photo so my best guess is that you had some cold nights of below 50 degrees in your area and zinnias hate cold nights. Are the leaves a bit purple? Do the flowers look faded? Too much water can also cause problems as zinnias need well drained soil as well as warm days and nights. Too much rain and water can damage their roots. My advice is to just wait it out and when summer arrives your zinnia should perk up. Cover your zinnias and other heat lovers like tomatoes and basil at night until temps are above 50 degrees. Keep growing, Marianne Binetti