I did my taxes with TurboTax about a week ago, but just now logged into the app. The app is telling me that I haven’t filed yet, but that’s impossible. I’ve already filed. I have emails

So there's no evidence you prepared the 2018 return?   If not, then you probably have multiple accounts without realizing it.

The email address you see on your notifications is not necessarily the correct User ID.  
You can have up to 5 accounts that use the same email address for notification purposes.  A User ID may be an email address, but it doesn't have to be.  It might be only part of an email address, or it can be anything at all.

See my answer below on how to troubleshoot.

If you prepared the 2018 return online and can't find it, and it's acting like it wants you to start one, then you likely have multiple accounts (User IDs) without realizing it and signed into the wrong one.  This is one of the more common questions in the forum.  

You can have up to 5 accounts that use the same email address for notification purposes.  A User ID may be an email address, but it doesn't have to be.  It might be only part of an email address, or it can be anything at all.

To get a list of your User IDs and recover account access, you can use the tool at the link below.  Try using your phone number first.   After that, if necessary, then run the tool on your email address(es) you can access.

NOTE: Before running the account recovery tool below, log out of all Intuit accounts including this user forum, or you might end up in a loop.  Then clear your browser Internet cache, close your browser, then reopen it, and go to the link below.  You may wish to copy this link so you can paste it into the new browser session.

https://myturbotax.intuit.com/account-recovery

If still no luck after running that on your phone number and email address(es) that you can access,  here's another method.
Go back to the tool again, but this time leave the data field blank, and choose the small blue link that says "Try something else", and it will look you up by SSN and other parameters.