My retirement agency stated that a loan I took out in 2020 was going to become taxable income for that year. How does this work if I already filed?

[MaryK4]

You would have to amend your 2020 tax return.  A loan that is in default is generally treated as a taxable distribution from the plan of the entire outstanding balance of the loan (a “deemed distribution”). The plan’s terms will generally specify how the plan handles a default. If it were taxable in 2020, you would receive a 1099-R which you can enter in TurboTax.

[macuser_22]

Loan form what type of retirement account?

Did you default or leave employment.

Do you have a 1099-R?   If so - what code is in box 7?

Any taxable retirement account   for 2020 was required to be reported to you on a 1099-R not later then Jan 31, 2021.