If you are a non resident, tax at source would have been deducted u/s 195 by your buyer out of sale proceeds and this TDS would have been reflected in your 26AS, the statement in which your tax details and high value (AIR) transactions are recorded. As you did not file IT return, you would have not offered the capital gains, as a result of which you would have received the notice. It is possible that your actual tax liability would have been less than the TDS and you might have been entitled to refund also. If the property was sold in the financial year 2013-14, you can now file belated return latest by this month end, after which you will not be able to file return voluntarily. You can compute your long term capital gains and file returns latest by this month end. Once you file the return, you can then proceed to respond to the notice in the efiling portal itself. As you purchased property out of sale proceeds, you can mention that the property was purchased out of sale proceeds of an existing property.