Airtel Broadband Fraud by generating a monthly bill of 35K.

I have been using airtel for more than 2 years now and I was using the plan for 698 rs rental and speed of 512kbps with night unlimited from 10 PM to Morning 8 AM.But suddenly from past two months I have started getting enormous bills and When i enquired about this in June I was told the plan is same but there must be some error with the billing department and would be rectified.No rectification was done and service request was closed.Then Again comes the month of July and I receive a bill amount of 35,000.I called up the customer care number of times to enquire about my plan and I was told that My plan is same as mentioned above.But then when I had a talk with one of the supervisors and he told my plan has been changed and that too without my knowledge.It was a horror as they said they have stopped the unlimited download from 10PM to morning 8AM and I was not even informed about the same.This is ridiculous I have taken a plan according to my convenience and then those people change my plan from the backend and expect me to pay the same.I would be more willing to go to the court for the fraud which airtel has done with me by changing my plan altogether from the back end without my knowledge and then asking me to pay the same.It is as good as if some person has taken an unlimited plan and it is changed to a limited plan without his prior knowledge.And on top of all this they ask me to pay the amount else they won't disconnect my connection.This is mental harassment.I am stuck with airtel and can't even use it as am scared as to how much they would charge me for that.I am not paying any amount then the rental i had to pay.I will move the consumer court for that matter because you people are the culprit here.I have all the original bills which was provided to me at the time when i took the connection and in that it is mentioned clearly that night unlimited usage.Also the recorded voices of all the customer care people who told me that my plan was changed by them without my knowledge.Please help and let me know what needs to be done.I don't want to waste my hard earned money like this.Thanks,Prerna