You’re talking about your 2019 return that was likely filed in mid-summer 2020, after the deadline was extended?
Did you mail it in or file electronically? Do you have a copy of that return?
First go to IRS.gov/refunds and use the “where’s my refund” tool. That will get you started. And depending on the answer, you might want to go to the IRS Taxpayer Advocate office at https://www.irs.gov/taxpayer-advocate
And if you used a tax-preparer, contact them first to help you with this search.