This comes from Elizabeth Fullington cracking her problem:


The situation is a PC gets rebooted and comes up in "Bitlocker" mode, asking for the encryption key.  Perform this:

  1. On another device go to myaccount.microsoft.com 
  2. Choose the "Devices" tile
  3. Search for the device that shows it is in Bitlocker jail
  4. Click on the Bitlocker link
  5. Click on the recovery key
  6. Click on the encryption key


This is a re-telling of what happened and may not be exactly correct, but represents how it works.


Also see this reference about how it occurs:


Finding your BitLocker recovery key in Windows (microsoft.com) 


And this in-depth report describes the details of what is occurring:


What causes BitLocker Recovery Mode? - Specops Software