TurboTax Online saves your data each time you change screens. So you can come and go. Normally to exit, click at the top on MY ACCOUNT, then choose "Save and Sign Out." If that doesn't work, then just go to a neutral screen where you aren't entering any info, and you can simply close the browser.
When you return, log in and go to the 2016 Tax Timeline and click the orange "Take me to my return" button to continue.