I've used Turbo tax every year for the past 6 years but for some reason this year my information did not pre populate when starting the app. Why hasn't it showed up?

You might not be in the same account.  It's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online. 

https://ttlc.intuit.com/community/prior-year-return/help/why-didn-t-last-year-s-tax-return-transfer-over-in-turbotax-online/00/26496