Chosen Solution

My 11" MacBook Air Mid 2013 suddenly turned off while I was watching a movie. Now it does not turn on with battery power, it only turns on when connected to MagSave power adapter. Why? Can it be repaired by me?

Download this gem of an app: CoconutBattery. It will show you the state of the battery and if it is able to be charged. Post a screenshot of the main screen so we can see it.