The poor thing looks like it is suffering from transplant shock. Once you disturb the roots you must give the plant protection from hot sun and cold nights and keep the soil always moist as it adjusts over that first week outdoors. Perhaps the soil dried out, a cold night hit  or you had a very hot day and the plant was not used to the heat. Keep the soil moist and out of the sun for a few days and it may perk up. It would have preferred to go directly into the ground as long as you had a spot with great soil. There is some hope that the plant could  survive and adjust but if it continues to decline and dies at least you still learned something about transplanting dahlias: Disturb the roots of dahlias as little as possible and don't set an indoor plant out in the hot sun or have it endure cold nights until the plant has "hardened off" or gradually become used to weather extremes. Keep growing, Marianne Binetti