Do you need the 2018 tax forms, or do you need to do your 2018 tax return?
If you want to do a prior year return, you can do it with TurboTax, but you will have to use the desktop software that you will download to your computer - the Online system is only for the current tax year (2019).
For instructions on how to do this, please see this TurboTax FAQ on how to file a prior year return.
If you just want the blank forms, you can go to the IRS website and download the forms and print them.
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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.
To get a copy of your previously filed returns prepared with online TurboTax https://ttlc.intuit.com/questions/1900748-how-do-i-get-a-copy-of-a-return-i-filed-in-turbotax-online