Microsoft have embedded in their Windows OS, a encryption feature called Bitlocker, it can be used to encrypt your hard drive/s if you so wish. It is turned on and off through the control panel under 'Prvacy & Security'.
To quote Microsoft's Information:
A BitLocker recovery key is needed when BitLocker can’t automatically unlock an encrypted drive in Windows. This key, which is a 48-digit number, is used to regain access to the drive.
A) You might be prompted for the BitLocker recovery key during startup, due to a security risk or hardware change:
B) You might need to provide a BitLocker recovery key for data or external drives, for example if you forgot the unlock password.
Even though you do not use the Bitlocker Disc Encryption, the proactive saving of the 'Recover Key' is of paramount importance. (Although the key can be found if disaster happens, but it is much easier if stored in the first instance)
Click here to go to the Recover Key Instructions Web Site