1. Did you ask the said person to vacate your flat?
2. Did you take the decision to stop paying the EMIs to the Bank on hearing from tayt person that he will not vacate your said flat?
3. Actually, you should have filed an eviction suit since it is your flat and he has trespassed in to your flat without your consent.
4. However, since you have already stopped repaying your lpan to the Bank and since your loan account has become NPA, the Bank will issue notice u/s 13(4) of SARFAESI Act for taking symbolic possession of your said mortgaged flat.
5. You can write to the Bank giving your consent to take over physical possession of the said flat for selling it and recover your dues.
6. Thereafter the Bank will apply before the local DM u/s14 of the Act for DM's taking physical possession of the said property by evicting that person for handing over to the Bank for selling the same.
7. After selling the said property, the Bank will refund you the amount collected by then, if any, in excess of your total dues.
8. You can also submit your bid to buy the said property in some other name or offer to pay off the entire dues on getting the sale notice after the said person is evicted by the DM.
9. However, in the entire process your CIBIL score will be spoiled and you shall not get any loan from any Bank who are the members of CIBIL.