First you need to be clear on the terms and conditions you signed while taking the education loan in the year 2008. Further, CSIS is for the loans disbursed after 01.04.2009. It seems yours was disbursed before that hence you are not eligible. Please contact Punjab National Bank if any internal circular was made by them in this regard.
If you were told that subsidy is to be given to you then a form might have got filled by you in this regard. please check all documents which were filled for the educational loan and check whether any such clause was signed by you.
The letter which is being sent by the branch manager asking to deposit the money is a SCN before the account is converted into an NPA. if the amount is not paid within the time as specified, then the account will duly be converted into NPA. Act and do the needful at the earliest.