I was told I already filed my tax return, but I didn't.

You may be a victim of Identity Theft — a common scam is for someone else to use your personal information to file a tax return and steal your refund.

If you can’t file your tax return electronically because someone has already used your SSN to file a tax return, you must file a paper tax return, along with form 14039 https://www.irs.gov/pub/irs-pdf/f14039.pdf

The IRS has a specialized unit dealing with identity theft - you can contact them at 1 (800) 908-4490.