I downloaded my 2016 tax return from the "previous taxes" page, but the file doesn't have the full return. It's missing the most important pages (1040, Schedule C, etc)

If you saved a PDF from the online home page and it did not have the entire return then you did not complete the return using the online program  OR  with that account. 

Did you use or finish with the Downloaded version instead ?   If so then only you have the return on the computer you used. 

Or you need to look for another account :    

Try these steps to recover your User ID or password:

1. Go to our account recovery page.
2. Enter one of the following:
   • Phone number (recommended)
   • Email address
   • User ID
3. Follow the on-screen instructions.

Once you’re done, we’ll either send you a code to help you recover your account, or show/send you a list of accounts associated with your info.

• If you’re still unable to recover your account, select Try something else on the Account Recovery page. We’ll ask additional questions to verify your identity.