***If your battery is no longer turning on, it's likely in sleep mode. The battery automatically enters sleep mode when not in use, depending on its charge level. To reactivate it, press the Power button for 15 seconds — a light (green, blue, or red) will indicate the charge level. For more details, watch the video.***
Link of the video : https://www.youtube.com/watch?v=CgdZYKcMt2M