Not Detecting Battery

You need to replace the battery. If you click on the apple logo on the top left, then click "ABOUT MAC" then Select the "SYSTEM REPORT" you will find a description for power on the left hand side and that will take you to the battery information if there is no information the battery has gone bad or may be disconnected all together from being dropped or some other related type of incident. You can check the connection of the battery by opening the bottom of the laptop and checking the connection of the battery to the logic board on the inside. If you are not familiar with the process, i would recommend taking it to an authorized apple repair center to have them take a look at it.