Vanmoof x3 what to do in case of deep discharge?

Hey guys, I got my girlfriend's Vanmoof x3 out of the basement today after we left it there for about 5 months after the whole debacle. When I tried to charge it, I noticed that the skull just lights up briefly, followed by a "bzzz" and then the skull disappears, and this happens in a continuous loop. I've already tried restarting the bike, but to no avail. Furthermore, the charger is showing a green dot instead of a red one. I'm worried the battery might be dead. Does anyone have any experience or ideas on this? Or maybe even have information about the status of spare parts.
Thank you, have a nice evening.

Ps: the cable is fine, I used it on my S3.