Q.My Crown of Thorns was in the house for the winter. Its leaves are all looking dead and the stems are poor looking. What could have
caused this? Also, can it be revived? The plant is also now outside. I live in Texas.

It's difficult to say without seeing the condition of the plant.
Continue to care for it and wait it out. You have nothing to lose.
This link will help refresh you on care.
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/cutting-back-crown-of-thorns.htm
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm