These are trial and error steps:
a) Bitlocker repair tool
b) Another operating system
See these links and bitlocker tutorials.
Create a bootable Ubunu flash drive > boot to the flash drive > see what may be able to be recovered using a different operating system
https://tutorials.ubuntu.com/tutorial/tutorial-create-a-usb-stick-on-windows#0
https://www.pcworld.com/article/2147063/linux-to-the-rescue-how-ubuntu-can-help-a-computer-in-distress.html
http://www.disk-image.com/faq-bootmenu.htm
https://video2.skills-academy.com/en-us/windows-server/administration/windows-commands/repair-bde
https://www.tenforums.com/tutorials/97390-use-bitlocker-repair-tool-recover-encrypted-drive-windows.html
https://www.tenforums.com/tutorials/97066-add-remove-change-bitlocker-password-context-menu-windows-10-a.html
https://www.tenforums.com/tutorials/38259-add-suspend-bitlocker-protection-context-menu-windows.html
https://www.tenforums.com/tutorials/96957-change-bitlocker-password-windows-10-a.html
https://www.tenforums.com/tutorials/36901-check-bitlocker-drive-encryption-status-windows-10-a.html
https://www.tenforums.com/tutorials/38508-suspend-resume-bitlocker-protection-drive-windows-10-a.html
https://www.tenforums.com/tutorials/97242-unlock-os-drive-encrypted-bitlocker-windows-10-a.html
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