Nestaway Technologies — Security Deposit has been deducted wrongly

I moved out of NestAway on 14 june and there was a supposed security amount of 26k pending. They deducted my 15k for some electricity bill that has not been paid since an year before I moved in and i like in 3 people flat so no one paid that bill and since I was moving out they deducted that amount from me. I have raised complaints tried to reach out to various ground staff but to no help.
Nestaway has tendency to annoy their customer base and irritate to the point where customer just gives in and let them have a money because no one has the patience to deal with this sort of behaviour.
I was not informed before deduction and was not informed about why whole amount was deducted from me.
further I was never informed that this is a bill I was suppose to pay because this bill comes under maintanince which nestAway takes care off.
I have no idea what is happening here and with covid and salary cuts 15k is a good amount for me which I can not afford to loose.
please help me out here