I didn’t file a 2015 return and now learned that I’m owed a refund. What can I do?

You have three years from the due date of the tax return to file for a refund. So you must file a 2015 return no later than April 15, 2019, to claim the refund. You can find prior year tax forms from the IRS at https://www.irs.gov/forms-pubs/prior-year.