2018 taxes filed by mail, but just realized now (July 2021) they were never received/not found in the system as ever filed. How do I proceed to actually file them?

Help! I filed my 2018 taxes filed by mail, but just realized now (July 2021) they were never received/not registered as having been filed. I completed them via TurboTax, and I think they were rejected for e-file (it was a strange tax year because moved back from abroad in the year), so that's why I printed and mailed them. I foolishly just assumed everything was OK (I knew I didn't owe anything). Just realized now I could log into the IRS (to check on a stimulus check) and realized they think I never filed for that year!! What should I do?

 

I'm afraid now to just print and mail again, and wonder if the problem is it never arrived, or there was a problem when it got there? If it got there, but something was wrong, would they have contacted me, or just marked it as not filed? Should I actually talk to the IRS? If so, how does one do that? I can't figure out a way to find a human when I call the line. Should I make an appointment at a physical office?

 

Just want to be sure I do the right thing! Definitely always tried (and thought I did) file my taxes! Don't know if I can trust just printing and mailing again? THANK YOU SO MUCH FOR ANY HELP! SO much appreciated!!!