Places to look for your BitLocker key:
On a printout you saved. Look in places you keep important papers.
Saved on a USB flash drive. Plug the USB flash drive in to 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 your Microsoft account. To get your recovery key, go to BitLocker Recovery Keys.
In your Azure Active Directory account. For work PCs where you sign in with an Azure Active Directory account, to get your recovery key, see the device info for your Microsoft Azure account.
Or ask someone for help:
Ask someone with administrator privileges on the same PC to unlock it with their key.
If your PC is connected to a domain (usually a work or school computer), ask a system administrator for your recovery key.
If you still can't get in, you'll need to reset your PC. Learn how.