Summarize:

You don't have to bring the chicken up to room temperature if it's been frozen, but make sure it's not frozen solid. Place it in the refrigerator for several (6-8) hours before reheating it to slowly bring the temperature up. If you are going to reheat the chicken immediately, place the frozen chicken in a water-proof Ziploc bag and run cold water over it until it thaws. You can also thaw it in the microwave on the "Defrost" setting. This will prevent the chicken from drying out during the reheating process. Different ovens take different amounts of time to preheat, so make sure the oven is up to the right temperature before you insert the chicken for reheating. Once the oven has preheated, place the chicken in the oven. If the chicken has been cut into small pieces, it may take only a few minutes to reheat properly. If you are reheating large pieces, like whole breasts, you may have to wait longer. Use a meat thermometer to check the internal temperature and make sure you're not leaving the middle cold.
Defrost the chicken if it's frozen. Cut the chicken into small pieces. Preheat the oven to 425 to 475F (218 to 246C). Reheat the chicken to 165F (74C).