Depends on what you are actually talking about.
1) you prepared your 2018 taxes using the Desktop software? Then your only copy of your tax file will be on that computer, and any backups you might have/ should have made. There is no direct link between the Online and Desktop software.
...or...
2) You prepared your 2018 taxes using the Desktop software, and are using "Online" software this year, and want to transfer-in the 2018 basic forms data into your new 2019 Online tax account. That can be done, as long as you have the ".tax2018" datafile on your home computer. Instructions here (expand the blue link in the FAQ "If you used the TurboTax CD/Download software last year" ) :
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-...
or 3) something else....then you'll have to explain
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*