That is a very timely question right now.  Because of COVID-19 the answer is different than it would have been without the impact of the pandemic.  Generally, you must file a tax return within three years of its due date to receive any refund due.  Since the 2016 return was due by April 15, 2017, April 15, 2020 was the normal deadline and you would lose your refund if you filed your return today.  However, because of the COVID-19 pandemic, the IRS has extended that deadline until July 15, which is the same as the 2020 deadline for filing your 2019 taxes.  You will still get any refund for 2017 and 2018 also, if filed now.  I urge everyone reading this to file your 2016 return soon if you have not done so.