Hello Have you tried to plug the disk back to the old machine then verify whether it could be accessed agagin? There is possibility that the disk has been encrypted before. We could open administrator command line and run "manage-bde -status D:" to check the protectors of the disk. If the passord option is listed, then we could have got a possword. In addition, we could check the Event Viewer\Applications and Services\Microsoft\Windows\Bitlocker-API\Management and check look for the encryption information of this disk both from the new machine and old machine.
You should get the time information when the disk was encrypted and what are the protectors. If you are using Microsoft Account to login, we could try to look for the recovery password from the following link. If you get the recovery key, you could acces the disk with the recovery key. https://account.microsoft.com/devices/recoverykey Finding your BitLocker recovery key in Windows - Microsoft Support If there is no any password or recovery key information, I am afraid we may lose the data of the disk.