Hey,
I had the same problem 2 weeks ago on my macbook pro. After talking to apple hotline 3 times, finally somebody knew what was going on, had to wait a week for an appointment at apple store, where they made sure my apple ID was restored (with a new password, so back in my control) (which I already had done with the guy on the phone) and to unlock the macbook he started the macbook in save mode (where it shows a lock and password field) then he typed something I didn't see and a long number showed up under the password field. With that he said, they have the authorization to unblock the computer.
So I left it there and after 1day I could pick it up and everything was like before 🙂!!
It was free, I still have warranty, I don't know how much it would be otherwise.
(before apple store app. actually had another app.with affiliated store (Gravis in Germany) but they had no clue and wanted to completely reinstall it, so if you can go to apple store!!)
For anybody still having this issue:
- Restore your apple ID with another computer
- make an apple store appointment (hopefully you live close by)
- make sure to bring the invoice and your ID! (If you don't have your ID/you're not on the invoice, they will unlock the device but also delete everything on it!)
For people that don't quite understand what this is (I didn't, I thought it's a virus and my other harddrives could be affected...). Somebody has somehow gotten access to your apple-ID password, logged in (possible on any browser, any computer) and "security" blocked your decive(s). That usually is an option you can choose to take when your device is stolen/lost, for your own safety, you can type a message to the finder (call me if found or whatever...) which the hacker has done in writing his email there. So it's not that bad after all, you won't loose your data if you do setps above! The hacker is not actually accessing anything.
Good luck, I hope that helped a bit!