Everything you reported above implies you have multiple accounts without realizing it. The emails you have don't exclude multiple accounts, since up to 5 accounts can use the same email address for notification purposes.
One other possibility: if you used the desktop version (CD/download) for your 2015 return, then it won't be in an online Tax Timeline.
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Here's how to troubleshoot multiple accounts:
To get a list of your User IDs and recover account access, you can use the tool at the link below. Try using your phone number first. If you try using your email address, and you have more than 1 email address, you should run the tool on each one.
NOTE: Before running the account recovery tool below, log out of all Intuit accounts including this user forum, or you might end up in a loop. Then clear your browser Internet cache, close your browser, then reopen it, and go to the link below. You may wish to copy this link so you can paste it into the new browser session.
https://myturbotax.intuit.com/account-recovery
If still no luck after running that on your phone number and each email address you have, then run the tool yet again, but this time choose the small blue link that says "Try something else", and it will look you up by SSN and other parameters.
Once you find the 2015 return, if it was prepared in Free Edition, the access will probably be locked until you pay for a 2016 product. We can tell you how to do that.
However, even if locked, there might be a link in the 2015 Tax Timeline under "Some things you can do" that says "View adjusted gross income (AGI)"
Or you can download a free 2015 tax return transcript at this IRS website. If you don't qualify for the transcript download, you can order one there to come by postal mail in 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript
