iCloud hacked and Macbook Pro locked

Based on IconDaemon's response, I removed the RAM and pressed cmd+option+P+R to reset the firmware password and rebooting back resolved this issue (since I had recovered my iCloud account and reset the password).

This may not be a complete answer, but off the top of my head, I would turn the Macbook off, then turn it back on while holding down Cmd+R to boot into Recovery Mode. Then open a terminal window, and as long as FileVault isn't turned on, you can change the password for any user using the single command resetpassword (which I personally feel is incredibly insecure, but that's beside the point).

Then, restart normally, log in as the user with the new password, and completely disable iCloud, or if you have the service secured now with an updated password, everything should be fine. Just to be safe, I would remove your iCloud account and then re-add it. That may be over-doing it, but it's just what I would do if it were me.

If worse comes to worst, just reboot into Recovery Mode holding Cmd+R (as stated above) and do a restore from your latest Time Machine backup.

If you have iCloud access securely-restored as you said, there may be a way to unlock remotely using FindMyiPhone (which also registers other devices) if that option is enabled on the computers you wish to unlock.

The same problem happened with me a pair days ago. But finally I was lucky and now my iMac, iPhone and iPad are unlocked and being safe.

Here is my story (use Google Translate or Google Chrome to read this in English): http://serdeles.livejournal.com/636852.html

If hackers didn't change password to your ID, it is cool. If they do, it's sad. First I thought they've changed my password and deleted my ID because when I tried to login in my Apple ID I was noticed there is no such Apple ID. But then I noticed in my iPad settings some unknown ID. Not mine. I used this ID as a login with my password. This trick helped. I changed back my ID. I logged in iCloud and turned off the Lost devices mode.

Good Luck! Hope, this information is useful.