Get an income transcript from the IRS ... that info will be on it. https://www.irs.gov/individuals/get-transcript
If the lender is "out of business" that means that some other entity now has the loan that you are still paying on. They can assist you with figuring the amount of interest you paid on the loan in 2020.
Take note that mortgage interest on your primary home, 2nd home, vacation home or any other "personal use" type of property is a SCH A itemized deduction. If all of your itemized deductions do not exceed your standard deduction, then the mortgage interest paid will have absolutely no affect on your tax liability. For the 2020 tax year the standard deduction is $12,400 if filing single or Married filing separately, $24,800 if married filing joint, and $18.650 if filing Head of Household.
@Carl Carl's advice is the easiest. another way is if you know your mortgage balance on 12/31/2019 and 12/31/2020 and the total mortgage payments excluding escrow deposits
mortgage balance on 12/31/2020 less the balance on 12/31/2019 is the principal you paid
not if you know the mortgage principal and interest payments made in 2020 just subtract the principal portion as determined above to arrive at the interest.