Any help would be appreciated

I have been paying on a credit card that had an original balance of $1100.00 I now owe $2,127.00. I have been paying every month at a rate agreed upon several years ago. Orchard Bank had agreed to close the account so I could pay it off. But they continued to charge interest, then over the limit and then late fees even though my payment was received 15 days before the due date. Last month they refused payment and sold the account to MCM. I have paid over $4,000.00 over the past 7 years on this original balance, getting nowhere. Does anyone have a suggestion as to what I can do and how to handle this letter I just got from MCM demanding payment in full. I'm on a limited budget, own nothing and have only small amount of money in the bank.