1. If there is criminal activity done in a bank a/c then the bank on instruction of the investigating agency can freeze the a/c.

2. So take information the source of freezing the a/c and then make your representation.

3. If your a/c has been frozen due to instructions of the Police then your mere willingness to reverse is not enough to open the a/c from freezing and in that even you will gave to apply before the Magistrate. 

You enquire from bank that why and on whose orders this has been frozen 

A bank in India can freeze a bank account - either current or savings - without the order of the Court, subject to the following conditions: Due to non-Compliance of KYC requirements by the account holder. ...

Upon receiving orders from SEBI, RBI, and Income Tax Authorities for freezing of the account of an account holder.

Conducting Unlawful Activities such as money laundering, terrorist funding, receiving suspicious payments from outside the country, etc.

Depositing huge amount of money in a way that it creates suspicion in the minds of the bankers.

When a bank freezes an account, the account holder will, still, have limited access to his account. He can check his transactions and receive certain deposits. However, he cannot withdraw or transfer his money to another account till the issue that caused the account to be frozen is settled completely to the satisfaction of the bank.

You may clarify from the bank about it and take necessary action as instructed by bank in this regard.

Sir/Madam,

please write it to your bank that you wish to reverse the money received in your bank account, which is out of your knowledge.

Due to this wrong entry of money, your bank account is frozen and that you are unable to transact. Also if possible try to extract the details of the person who has wrongly credited this amount to you and mark a copy of the bank letter to that person too. 

Write to bank to de freeze account 

2) if bank refuses seek court orders to direct account to de freeze the bank account

Dear Sir,

According to Reserve Bank of India (RBI) rules, it is mandatory for bank account holders to provide their PAN or Form 60. In a circular issued in December 2016, RBI instructed all banks to disallow debit transactions in accounts where PAN or Form 60 (in terms of Rule 114B of the Income Tax Rules) had not been provided.

• Debit freeze for non updation of PAN/Form 60 may be done only in the following cases:
• Account balance is Rs 5 lakh or more.
• Total deposits made after 9 November 2016 exceed Rs 2 lakh.

Unfreezing debit freeze
In order to unfreeze the debit freeze on one’s account, the account holder must forthwith furnish PAN/Form 60 (as applicable) to the bank.

Visit to the branch
The account holder can visit the nearest bank branch and fill up a designated form for updation of customer details. Self attested copy of PAN or Form 60 shall be attached to the form and submitted. Original PAN should also be carried along at the time of submission

You need to find out the reason for freezing then should file application to defreeze. You need to approach banking ombudsman or court to defreeze it

See you can go and talk to bank and give written complaint about the said transaction and requesting bank to un freeze you account if on your request bank fails to do it file complaint before the banking ombudsman.

This is new techniques of putting some one in legal trap.  

Someone might have deposited the money in your bank and made complaint to police stating it as a part of some transaction and you have cheated that person. 

Police might have requested to bank to temporarily freezes the although it is illegal.

You may contact local police in this regard or enquiry from your bank. 

Inform further development for further advice. 

1. File an Affidavit /Indemnity Bond, declaring that deposit of 12000/- is not done by any of your parties and same amount may be transferred to their suspense account of bank.

2. Declare that you indemnify the bank against any claims or losses and request them to defreeze your bank account.

3. IF the Bank still fails to defreeze the bank account, THEN make written complaint to the Bankinig Ombusman and the RBI.

 if there is no problem by the bank and they have not filed any criminal complaint against you then you can file a mandamus writ petition directing the bank from the Highcourt to unfreeze your account and in case there is any suspicious credit that may be investigated and for that purpose Bank can file FIR against you and the matter will be taken up by the police if you are sure that there is no transaction of malicious nature then this is the best way out to unfreeze your account

Bank cannot freeze without issuance of notice and seek clarification. Have to file civil suit for direction to unfreeze. And before that mail to bank, if any unauthorized transaction reported, can reverse it. And only that 12,000 rs transaction. 

Please enquire from your bank why the account has been freezed .

bank can freeze a account  either current or savings  without the order of the Court, subject to the following conditions .. Upon receiving orders from SEBI, RBI, and Income Tax Authorities for freezing of the account of an account holder.

You should send legal notice to bank for unfreezing your account and reverse the transaction on which they have mentioned fraud activity.

If they refuse to unfreeze the account then you have to file suit against bank for blocking your money without any evidence of fraud.