How to address & write a letter to a bank

I will be very pleased if someone could help me to write a letter regarding a problem faced while I tried to withdraw money from an ATM. What actually happened was, I went withdraw money from the ATM- A ( which belong to xxx bank ). I Entered the daily withdraw-able amount of 20,000 Rupees. It Displayed "... please recive you money... ". So I was waiting to receive the money, but like after 15 seconds It displayed a message saying something like your transaction cannot be completed ( I dont remember the exact message it displayed ) . however no receipt was issued and it ejected My Visa Card. Then I tried to withdraw for the second time to check if i could get a lesser amount ( 5,000 ). This time it gave me 5,000 Rupees and issued the Receipt as well. So, i thought that the ATM ran out of money and because of that I could not withdraw My first Transaction of ( 20,000 Rupees ).

Then I went ATM-B ( which belong to yyy bank ) and tried to withdraw the remaining 15,000 Rupees. but this time the ATM-B Displayed a message saying that I entered an amount greater than the amount I have in my account. so I finally decided to check My Account balance, It showed that i don't have enough money to withdraw. which means the first ATM Machine thinks that i have already received 20,000. ( which i did not receive ) and it did not gave my Visa Card back to me. so today I went to the bank and explained what happened; they gave me my card back and told me to write a letter. then only if they found the correctness of my explanation they will give me 20,000 ( less the 5,000 i withdrew from ATM-A ).

They told to submit the letter today :-/ please help me for gods sake. :-(

Looking forward for a reply.
Thank You For Your Time.