mac book is locked

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That's a firmware password. You set it in OS X Recovery and you should know that password in order to start your Mac from a DVD, USB drive, in OS X Recovery, in any OS X startup mode except the normal one...

Only Apple knows how to reset it in new Macs (in old Macs you could extract a RAM slot and that firmware password was resetted), so take it to an Apple Store or reseller.

It's also important to ask if you got a new or used Mac. If you are going to get an used Mac, check first that there are no firmware passwords

It's a Firmware locked and it does happen mainly with Lion and Mountain Lion. It supposed to be used as a protection in case someone stole your MacBook, but with Lion and Mountain Lion, sometimes, this lock just shows up as a bug. Some used Mac / Second MacBook Pro might have it because the previous own put in a lock to prevent if from being stolen and they forgot to remove it before selling it away.

I did run into the problem but luckily when it happened, it was just a bug. It might not work in your country, but the way I deal with it was I triggered a serious malfunction to the hard disk and gave it to the Apple Store. They didn't ask for my ID card. They just took the computer in, reset the Firmware Password, and replace a new Hard disk.

CrimsonCage, after resetting the password there shouldn't be any need to replace the hard drive, or did the Apple Store detect something damaged in the hard drive?

I ask you because it's the first time I read that this problem is related to the hard drive, when the password is related to firmware

Since my MacBook Pro 17 inch 2010 doesn't have an Apple Care, I usually upgrade it by myself, and if a problem occurs, I fix it by myself.

One day, I installed a Mac OS X 10.8 Mountain Lion via App Store and the Firmware Locked showed up. I decided to try and replace some files by connecting my MacBook Pro's Hard Disk to my MacBook Pro 17 inch (Late 2011) via SATA to USB cable. After that, not only do I have this firmware lock, but also my MacBook Pro cannot detect any OS X. If you press Option, it will enter this Firmware Lock Screen, and if you don't press the Option key, it will enter a folder with a question mark.

Since I couldn't do anything about it, I decided to send it to the Apple Store, and after a few weeks, they called me and said that they needed to replace the hard disk. They stated that 'this was supposed to be a normal Firmware Lock. However, when they entered the password, they got a kernel panic. The one solution that is working is when they change the Hard Disk. That's why they didn't ask for my ID card. Instead, they successfully reset the Firmware password and replace the hard disk at the same time. It's cool because instead of paying nearly ¥6000, I paid a little over ¥2800.