There are a few possible reasons.
If you were using TurboTax Online Free last year, that version of the software
doesn’t include the ability to transfer data to future years. For 2017, you can
add +PLUS to your Free return that will allow you to do this in future years,
but +PLUS was not available in 2016.
If you were using TurboTax Online
Standard or above last year, then perhaps you have multiple user ids and you
are logged into a different one that you used last year. Or you may be in the
right account, but in the wrong return. This FAQ may help: https://turbotax.community.intuit.ca/replies/2648026