HDFC Bank — hdfc bank salary account negative balance

Address:

Chandigarh, Chandigarh, 160035

Hi, i had salary account in hdfc 2 years back in one company. I was doing my internship and after my internship i left for my further studies.
The bank converted my salary account into savings account & charged monthly.. Now its 9200 negative balance. Without my permission how can the bank convert my salary account into savings account? Even they are not intimated verbally... According to rbi guidelines on savings account banks not charge even its negative balance.
I approach branch, but they behaved rudely & refused to close account.
Plz do juistice to me. 9200 is huge amount to me. I am not able to pay this amont. What is the solution?
What is the action i can will take against this bank.