Bitlocker is a full volume encryption technology prevelant on Windows-based computers. If the computer BIOS thinks that the computer has been or is being tampered with, the Bitlocker technology may lock the volume to prevent data from falling into the wrong hands. The computer is then only accessible with the Bitlocker key that was set (automatically) when the computer was originally set up. It is stored in your Microsoft account - here are instructions to help locate it.