I was nearly finished with my return yesterday. I sign in today and nearly all my data is gone. Customer service says this isn't possible.
Did you forget how to access your account? Online software autosaves so whatever you entered should still be there if you are in the same account.
You have to access your own account and/or print it for yourself using exactly the same account and user ID that you used when you prepared the return.
Many people have multiple TT accounts and forget how to access them. Log out of the account you are in now.
Account recovery
https://myturbotax.intuit.com/account-recovery/
https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password
Or did you use the desktop version of TurboTax? If so, the files are on your own hard drive or any backup device you used like a flash drive.