Bajaj Finserv — credit score was hampered

Address:	Shantigar, North 24 Parganas, West Bengal, 743123
Website:	bajajfinserv.in

I purchased a product using bajajfinserv and the emi tenure was 5/6 months.
Not a single penny from my bank account was reduced and this was totally their fault. I did try to reach them via email and telephonic conversation
They always said me to pay my debts and when ever i asked them what about my credit score the were totally shut.
Now why do i suffer from this? Without any reason my credit score is hampered from 850 today it has strikes to 579.
Why??
Am i responsible???
Please help me i want my credit score to update.