@Own

1. Check voltage on battery and write a message here what voltage it is.

Things you can do: Try another charger. Double check cables.

Things it can be. There may be error in the battery electronics and when disconneted it will give blinking 0% for the battery status. That is also the case if the smaller 4-pin cable that connects to the big bike cable is disconnected and only connects big + and - from there. The real battery status is. If voltage is 69,7v then battery is full. If voltage is 51v I believe it is empty.

2. Batteries work that if you don't charge them for 1-2 months they may self discharge very slowly but time is the enemy and for too long and left at a low voltage 0-10% you may end up with a dead battery.

Recommendation is. Always charge full when you get a low battery level and also every 1-2 months at least to be safe. 

In your case. Leave it to your dealer. If he can't help you then sadly you have just lost a bit of money. If you know how or if you have a friend good with electronics and really knows what this means:

17s battery, test one series of the group of these and if voltage is 0-4,2 volt 3-4,2v is normal voltage levels and below is odd, bad level. The lower voltage for the one group the worse it is. If they are 0 the battery is with almost 99,99% scrap junk.

I believe in this case that your voltage for any one of the 17 series groups probably sits between 0-2,5v  each. By charging it up manually each group 4,2v max voltage or all at once 17s = 69,7 volt. I would do the later. Then my belief is that the bms will turn on battery.

BMS = battery managment system and it checks voltages and if they are wrong it will turn the battery off pretty easely explained)

Open it up. Check voltage. Charge manually if you know how. Now your battery works again.

Worst case if the battery is dead... you can DIY a completely new battery, with better power and more range 😀

Of course you can salvage some cells, which you can sell or re-use.