It is not your computer that has been hacked, but your Apple ID. After gaining access to that, they locked your Mac with the Find My Mac function: Use a firmware password on your Mac - Apple Support
These instructions can get you out of this, but you'll probably need Apple's assistance with that: If you think your Apple ID has been compromised - Apple Support
For future reference: be sure to activate two-factor authentication for your Apple ID: Two-factor authentication for Apple ID - Apple Support
Aside from that also set your own Firmware password as soon as you are able to user your Mac again.
There is only one way (see the last section in the first link I gave): contact Apple Support or get in touch with a local Apple reseller or Apple Store. They'll want you to bring your Macbook with proof of ownership.
My macbook air is HACKED by Help.apple@gmx.com and asked to wire money $50 . Sick