I'm not sure exactly what you mean by "won't bring it up." TurboTax will not automatically find a tax return data file (the .tax2018 file) that it did not save on the same computer. To open the .tax2018 file that you downloaded from TurboTax Online, on the first screen when you start TurboTax, click the blue link that says "Find a Tax File." Then navigate to where you saved the .tax2018 file, select the file, and click the Open button.
After you have saved the tax return from the program on your computer, TurboTax will find it automatically from then on.
If your .tax2018 file contains a state tax return, you must install the state on your computer before you can open the downloaded file. To install the state, start a dummy new tax return, then on the Online menu select Download State.