How to Unlock My BitLocker-Encrypted Disk

This has nothing to do with your Mac or Mac OS. I'd suggest searching on a Windows forum or go to MS for assistance.

With a Mac, yes you can encrypt your data however NEVER EVER lose the password, if you do and forget it then your data is history. Apple warns users about losing the password (MS probably does the same) however there are those that either forget or ignore the advice and then when they need the password and don't have it become very frustrated. This is exactly why you should use a password manager such as 1Password on your phone and tablet and even your MM. I suggested 1Password, there are others however it is critical that you manage your logins and passwords for all the accounts you are responsible for.

Encryption exists for one reason only: to prevent access to data by anyone who does not have the cypher key.

Without the key, you cannot access the data. If you feel the security of your data warrants the use of an extreme security measure as encryption, then you must take responsibility for securing the key(s).

Apple has no way to unlock encrypted user data. They can provide the means to encrypt and provide the key at the time the data is converted, but they cannot decrypt that data without the key after the fact.

Unfortunately, this situation will be a learning point for you.

Don't give up. Try looking through your old "thumb drives," or perhaps you printed off the key, and it is in amongst some papers. Is it possible you took a backup of the data that isn't encrypted?

Unfortunately, as others have said, the key is needed, and there are no easy fixes. If there were easy fixes, then the encryption procedure wouldn't be valuable. Good luck.

Without having the key to access the drive, accessing the data is impossible by design. If there was a work around to decrypt the data, then there is no point to encrypting it! That is the whole point of encryption.

Apple doesn’t implement Microsoft BitLocker, and Microsoft BitLocker isn’t all that common a topic around here. Which means we’re not the best audience for this technical quest, even though there is a Mac involved.

BitLocker and most any storage encryption is explicitly intended to block exactly what you are attempting; data access without the decryption key.

The recent BitLocker cracks I’m aware of (bitpixie and related, including what was announced at CCC a few months back) all (still) need (brief) access to the PC involved. (With access to the PC)

I fully expect you’ll be told “nope” by the Windows-knowledgeable folks, though.

There are also two types of BitLocker encryption......one is hardware based, the other is software based. If it is hardware based, there is absolutely nothing you can do because the TPM on the Windows laptop held the only key.....it is rare to see the hardware based encryption since it can only be initiated under special circumstances.

If it was using the Bitlocker software encryption and you had the recovery key, then there is a possibility of unlocking the drive either with a third party macOS app (assuming it is still available), or using another non-Apple PC, or from a professional recovery service.

Also, you should have backups of that old Windows PC installation. So that is another way you can get access to your data. If you don't have backups, then the data was not that important to begin with. I really hope you have frequent & regular backups of your Mac since there are so many new ways to permanently lose access to the data stored on the internal SSD of the recent Macs due to all the hardware, software, and security changes.

Back up your Mac with Time Machine - Apple Support

The software is already installed on your Mac. Just add an external drive.