You don't calculate a 2017 AGI. It was calculated on your prior return. You have to view it either on your 2017 return, IRS tax return transcript, etc.
Assuming you used Online TurboTax for 2017, I'll tell you below how to look for multiple accounts and regain access.
Try to find the online return first using the steps in my Answer below. If push comes to shove, you may be able to immediately download a 2017 return transcript at the IRS website at the link below. If you don't qualify to open a free IRS account there, you can order one to come by postal mail, but that normally takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>
