How do I find out why a return was rejected for a previous year?

What tax year?   If you used TurboTax you would have received an email that told you why the return was rejected.  And.....you can no longer e-file an earlier year now anyway.  The only way to file it is by mail.   If it was rejected for AGI---the most common reason----the AGI no longer matters since you only needed to enter AGI to e-file.  If you have a tax return for 2019 to 2021 that was rejected and you need to file it....then print it, sign and date it in ink and mail it in.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.