The rule of thumb is that you shouldn't let the chicken stay in the danger zone -- between 40 and 140 -- for more than 2 hours. Too much bacteria, and the bacteria multiplies zillions of times during this period. Proper heating and reheating will kill foodborne bacteria. However, some foodborne bacteria produce poisons or toxins that are not destroyed by high cooking temperatures if the food is left out at room temperature for an extended period of time.
If it sat out unrefigerated and then sat in a crockpot at lower than normal heat, the chicken will have stayed in the danger zone for way more than 2 hours.
I would never do this. Chicken breasts are another story, I suppose.
For the record, the USDA considers cooking any frozen meat in a crockpot to be unsafe. They are hypersensitive, perhaps, but a whole frozen chicken just seems like asking for trouble.
Cooking a turkey overnight ina very slow oven is widely acknowledged to be unsafe. It's a recipe for food poisoning.
Less is not more. More is more and more is fabulous.