BitLocker Drive Encryption will lock an encrypted drive and require recovery in these situations:
The computer is operational, but BitLocker has detected a possible security risk during startup.
The computer is operational, but the BitLocker startup key or PIN is lost, or the startup key is damaged.
The computer is not operational (for example, a power supply problem), and you have transferred its hard disk to a different computer.
To unlock the drive and regain access to your files, you will need the BitLocker recovery password. If you also encrypted data drives, you will need to have access to the drive that Windows is installed on as well.
When you start the computer, BitLocker will interrupt startup and display a message informing you that the drive Windows is installed on is locked. Locate your BitLocker recovery password, and then follow the instructions on the screen.
If you have an encrypted data drive that is locked, you will need the drive that Windows is installed on to be operational so that you can unlock the data drive. You'll also need the BitLocker recovery password for that data drive, which is not the same password as the one you use for the drive that Windows is installed on.
Before removing the hard disk from the inoperable computer, check the host computer to which you're transferring the disk. The host computer must be running Windows and have BitLocker available (even if BitLocker has not been turned on).
Shut down both computers.
Remove the hard disk from the inoperable computer and install it in the host computer.
Start the host computer and log on.
Open Bitlocker Drive Encryption by clicking the Start button , clicking Control Panel, clicking Security, and then clicking Bitlocker Drive Encryption.
If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
The transferred drive will be indicated in the list of drives as a Locked Volume. Click the Unlock volume link, and then follow the instructions on the screen.
Some BitLocker features and settings can be enabled by Group Policy settings.