What "old" tax return? What tax year?
Many people have multiple TT accounts and forget how to access them. Log out of the account you are in now.
Or did you use the desktop version of TurboTax? If so, the files are on your own hard drive or any backup device you used like a flash drive.
Note: Right now you will not be able to purchase PLUS in order to access a prior year return, until sometime in December when the transition from 2018 to 2019 is complete.
If the prior return was prepared in Online TurboTax, and if there is no evidence of it when you log in and look in "Your Tax Returns & Documents", and if you have used the Account Recovery tool and cannot find another account, you can try using the alternate "Try something else" (described below) which uses your SSN and a series of questions to look for another account. I'll also tell you below how to phone TurboTax Support if necessary.
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.
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.
If you can't find evidence of the past return after all that, here's how to phone TurboTax Support during business hours:
FAQ: What is the TurboTax phone number?