I assume you used the Desktop program? As long as you have the data file ending in .tax2018 you have your return. And of course any pdf files won't be affected. You can install the 2018 program again. After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW TAX RETURN)
Then go to FILE -Open and find your return.
Mac System Requirements
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
Thank you for your response! I used the CD version of premier (not desktop). Intuit told me to move everything on my computer to a backup drive through time machine. I did this and now everything has been extremely slow ever since. Been a Turbo Tax user for 20 years and never had an issue until this year. I did not look at the minimum system requirements and they said doing this was the only way I could use the CD version that I had already purchased. I want to put everything back on my mac but not sure how to without losing something. Thank you!