You will need to log into the same account that was used last year. If you used our Federal Free product you will need to purchase PLUS benefits to retrieve that information (PLUS benefits must be purchased yearly in order to receive benefits) or you can go to https://www.irs.gov/individuals/get-transcript. If you purchased a product last year, your AGI will import to this years return or it will be on your Tax Timeline if you chooses not to import.
If you believe there are more than one account, you may want to try the account recovery process:
https://myturbotax.intuit.com/account-recovery/
Take a moment to look over this FAQ to ensure you are using the correct field for your 2015 AGI.
https://ttlc.intuit.com/replies/3301618
Follow this FAQ if you have access to the TurboTax Online account you used to file last year.
https://ttlc.intuit.com/replies/5574744