My Macbook Air just locked-up, and there was a message saying that Apple did it -- that my identity had been stolen -- was that Apple or a scam?

Scam: Phony "tech support" / "ransomware" popups and web pages - Apple Community

This may be due to somebody having accessed your Apple ID and having locked you out via the "Find My" app. Try to log back in to your computer, and follow Apple's guide on what to do if your account has been compromised: If you think your Apple ID has been compromised - Apple Support

The way I see it, you did the right thing to hang up.

Cross-check the number you dialled with a reverse search. Google the number, or visit the Apple Global Customer Service Telephone Number List (Contact Apple for support and service - Apple Support) to see if the number that you dialled matches any of those. Call Apple yourself to confirm if you still have doubts.

Visit a website that shows you if your account has been compromised (such as HaveIBeenPwned?) and if you see any information leaks, change your passwords on those websites, as well as all other websites on which you use the same password. Apple Keychain also has compromised account alerts.

I'm pretty sure Apple would never bar you from accessing your devices, without having previously informed you about it.

Hope this helped!

-flef

It is a scam.