Have you or someone else enabled BitLocker Drive Encryption? You can find this in Control Panel.
A bit from the Help Section regarding BitLocker..
'During computer startup, if BitLocker detects a system condition that could represent a security risk (for example, disk errors, a change to the BIOS , or changes to any startup files), it will lock the drive and require a special BitLocker recovery password to unlock it. Make sure that you create this recovery password when you turn on BitLocker for the first time; otherwise, you could permanently lose access to your files.'