Incorrect electricity bill

It's very serious issue. I got 50000 electricity bill of 8 years. I was told in January that your meter is being connected from prepaid to postpaid smart meter. I had ₹600 deposited in my prepaid and I was told that your ₹600 of prepaid will remain deposited in postpaid. I have just received a bill of ₹50000 whereas I had kept my prepaid recharge regularly and on top of that I had ₹600 deposited, now where did this bill of ₹50000 come from? I am in great mental agony. While installing the meter it was said that your ₹600 will remain deposited in postpaid and after that you will have to pay the bill that comes. If I had known earlier that such a huge bill would come then I would never have got it done from prepaid to postpaid.