At start-up, there are 2 way's a user may experience "Bitlocker."
The first (as shown below), is when Bitlocker actually locks the drive due to suspicious behavior. This could be caused by a number of things ranging from unexpected shut downs, to tampering with the hard drive. Bitlocker locks itself to protect the user's data. If you are receiving this message, Desktop Services should be contacted in order to unlock the drive.
The second (as shown below) is within the Start-Up Repair menu. At this point, Windows has identified that something caused the system to shut down unexpectedly and it is trying to resolve the issue. In order for Start-Up Repair to run on a system that is Bitlocker encrypted, it will prompt for the Bitlocker recovery key. If you are receiving this message, often restarting the PC, and selecting 'Start Windows Normally' when prompted will resolve the issue. This should allow the user to resume working with little interruption.
*It is possible that this error message is happening due to the a software or hardware issue (i.e. blue screening). If this is the case, please note it is NOT a Bitlocker issue, but a system error.