RE: Latitude 5175 Bitlocker issue after update BIOS 1.0.20 -> 1.0.24
Bitlocker encryption must be suspended before you begin BIOS flash. If you have not, that is when you are prompted for the recovery key.
When you enable BitLocker, you can create a recovery password (48 characters) and a recovery file. If you saved the recovery file somewhere, use that to unlock and decrypt. Reformatting and reinstalling the operating system is the only go, if you do not have the recovery key.
Please review http://dell.to/2remlwW for more information. This has some helpful links as well.