Battery empty (light on)

Hello, my sister left the light on the Opel Adam and now nothing works anymore. The door only opens mechanically, no clapping at the ignition, there’s everything. I connected a charger from the neighbor overnight and there’s nothing at all. Still 5.6 V as at the beginning. Have Lixht and everything switched off from the inside of course. When charging at the back window the light is always rhythmically very easy, is that normal? I have nothing from the battery Is the battery broken and mjss changed or do I have to disconnect EVERYTHING from the battery? Is that the reason why the battery does not charge, that I have not disconnected anything but connected the charger directly? I am grateful for help. LG