Place the thawed chicken in the slow cooker. Add the black beans, corn, bell pepper, ranch seasoning, cumin, and garlic powder. Spoon the salsa evenly over the top.
Cover and cook on low for 4 to 6 hours or on high for 2 to 3 hours, until the chicken reaches 165°F in the thickest part and is tender. Cooking time varies by slow cooker and the thickness of the chicken.
Transfer the chicken to a plate and shred it with two forks. Return it to the slow cooker and stir until coated with the salsa mixture.
Stir in the lime juice. Taste and adjust seasoning, then sprinkle with cilantro if desired. Serve in tortillas or over rice.