I've been a user for about the last 8 years, consecutively. This week, when I signed in, none of my previous information was available (or my spouse's). Where did it go?

Are you positive you are in the exact same account and used the exact same user ID?   Lots of people have multiple accounts and do not realize it---and you can have multiple accounts even with the same email.  Try the recovery tool:

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.

https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have

Thanks for the reply.

All of my TurboTax activity has been via the online application. I've only used this one, same email address to login every time, and the TT phone reps (x2) confirmed that there weren't any other IDs/accounts associated with that email.

Just now, I tried to use the account-recovery link that you posted, and I chose the verify option which takes longer... so, when submitting that form just now, TurboTax then posted a message: "There's a problem on our end."

Not sure how to proceed, except to just try waiting a bit and then swallow this huge pill and "start over." Phone support has been polite, but unhelpful, and there doesn't seem to be any other avenue to pursue a solution.