I can't find my federal return transcript. I'm locked out of my IRS account to download my previous transcript.

If you are a U.S. citizen or resident, and you filed a personal income tax return, you filed Form 1040, or 1040 SR or 1040X. If you are a nonresident, you file form 1040 NR. The IRS has you verify one tax year so you can have access to your account. If you have a previous tax return, for example, tax year 2018, use that to enter the information requested by the IRS. Make sure you enter the tax year as 2018, if using a 2018 income tax return.

To access your TurboTax prior year income tax returns, see How do I access a prior year return?

What should I do if I don't have a 1040 form or any tax forms from prior years to reference? I took my taxes to a tax agency last year the first time I had to deal with taxes. Whether the papers got lost in transit or I misplaced them is there anyway I can obtain my AGI? I currently only have my "2020 W-2 and Earnings Summary" paper.

If you filed any return in the past then you need a copy of it so if you used a paid preparer then they should have either given you a paper copy of the return OR  sent you an electronic copy.  If you don't have either then ask them for another copy.