cgsecurity.org

As I don't know how an Bitlocker encrypted partition appears in TestDisk or if it appears at all, you should duplicate the disk using ddrescue as described in the manual.
Afterwards you would completely decrypt the partition using your password.

With the encryption being gone you can use any tool ...

I have an external Bitlocked 500GB drive that now shows as unallocated and lost its drive letter.

Using trial versions of Diskgenius and Iboysoft, I find the folders and files are still present (once I have entered the bitlocker key). I'm also able to download -pending size restriction in the trial ...