BitLocker is a device encryption feature of Windows.
Important: If your device is asking you for your BitLocker recovery key, there is no "back door," there are no workarounds, and Microsoft support can't provide you with the missing key or create a new one for you. You will need that 48-digit key to unlock your device.
Here are the places you can check to locate the key:
In your Microsoft account: Sign in to your Microsoft account on another device to find your recovery key. This is the most likely place to find it.
Note: If the device was set up, or if BitLocker was turned on, by somebody else, the recovery key may be in that person’s Microsoft account.
On a printout you saved: Your recovery key may be on a printout that was saved when BitLocker was activated. Look where you keep important papers related to your computer.
On a USB flash drive: Plug the USB flash drive into your locked PC and follow the instructions. If you saved the key as a text file on the flash drive, use a different computer to read the text file.
In a work or school account: If your device was ever signed into an organization using a work or school email account, your recovery key may be stored in that organization's Azure AD account associated with your device. You may be able to access it directly or you may need to contact their IT help desk to access your recovery key.
Tip: During COVID we have seen a lot of customers who were suddenly working or attending school from home and may have been asked to sign into a work or school account from their personal computer. If that was your experience too, then it's possible your work or school has a copy of your BitLocker recovery key.
More info about BitLocker recovery keys