Reasons the Xbox 360 controller won't connect to an Xbox 360 include weakened or depleted batteries, wireless interference or a malfunctioning device, according to the Xbox website. Trying to connect more than four controllers to a single Xbox 360 also results in a failed connection for each subsequent Xbox 360 controller.
Disconnect the microphone (if applicable) from the controller. Press and hold the "Guide" button on the center of the controller to open the Guide screen. The amount of battery life is displayed in the battery bar on the top of the Guide screen. If the bar is blinking red, add two fresh AA batteries to the controller to see if that helps maintain a connection. When removing the old batteries, check the battery pack to see if the connectors are bent, which will cause an unstable connection even with new batteries.
Move any other wireless devices further away from the Xbox 360, as appliances such as microwaves or cell phones may cause interference. Move closer to the Xbox 360 if rearranging appliances isn't an option. If the Xbox 360 controller still won't connect, the controller may be malfunctioning and need replacement. If possible, try connecting the controller to a different Xbox 360 to see if the problem persists. If so, this indicates the controller is the problem.