Are only the flowers wilting, or are the leaves wilting also? Your plant may simply be in shock. Another possibility is that if your plant was NOT blooming before, it may simply be that you are viewing your plant late in the day when many hibiscus have "tired" looking flowers. Most hibiscus flowers only last one day, and tend to be at their best in the morning.
Be sure your plant gets adequate water, especially if it was recently taken out of a pot and planted in full sun. Its roots need time to grow out from the root "ball" into the surrounding soil where they can scavenge for minerals, and especially, water. Give it a chance and I think you'll be rewarded. Hibiscus are quite resilient plants.