Preheat the oven to 425 degrees F. Spray a 9×13 inch baking dish with cooking spray.
In a small saucepan, melt the butter over low heat, then whisk in flour. Add a ½ teaspoon of salt and a ½ teaspoon of ground black pepper. Slowly whisk in the milk a little at a time. Cook while whisking until the mixture has reached a creamy consistency. Whisk in the ranch dressing. Remove from heat.
Add the chicken in an even layer onto the bottom of the baking dish. Pour the prepared ranch sauce over the chicken. Top the sauce with half of the cheese and then the tater tots.
Place into the oven and bake at 425 degrees F. for 15-20 minutes until the tater tots are crispy and golden. Remove from the oven. Top with the remaining cheese and crumbled bacon. Return to the oven and bake for about 5 minutes, until the cheese has melted.