Our 2019 tax return was rejected and we didn't know it. How can we fix it?

It is too late to e-file a 2019 return---in order to file it you will need to print, sign and mail it in.   You do not say why it was rejected.   If it was rejected for AGI, that does not matter now since you only needed AGI to e-file--and you cannot e-file. 

 If you need to make any other changes---it gets trickier:

 If you have any changes/corrections at all to make to your 2019 return, you now have to complete it using desktop software, which you must download to a full PC or Mac  (not to a mobile device).Then you need to print, sign and mail it.

https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online

If you already paid for your online software you can ask customer support for a download of the desktop software.

They are available from 5 a.m. to 9 p.m. Pacific time Monday-Friday.

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

If you are getting a refund, there is no penalty for filing late.  If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe.  Only the IRS will calculate this—TurboTax will not calculate it.