Hi, Faith Hunter
A Windows or firmware update can change the TPM, Secure Boot, or startup configuration that BitLocker checks. Restoring the previous startup configuration may allow the TPM to unlock the drive again, but it will not help if the recovery key itself is truly unavailable.
Besides the solutions provided by the Q&A Assist, here are some suggestions you can try:
Shut down the computer completely. Disconnect USB drives, SD cards, docking stations, and external hard drives, then restart it.
If you are comfortable entering BIOS/UEFI, check whether TPM (sometimes called Intel PTT or AMD fTPM) and Secure Boot are enabled, the boot mode is UEFI, and Windows Boot Manager is first in the boot order. If the update included BIOS or firmware, contact the computer manufacturer for the correct settings for that specific model. Firmware support is provided by the device manufacturer.
Do not clear the TPM, reset Secure Boot keys, or make random BIOS changes, as this can make recovery more difficult.
To request live assistance, open Contact Microsoft Support, describe the issue, select Get Help, then Contact Support. Depending on your location and availability, chat or a callback option may be offered.
Thank you for your patience in reading, I hope this information has been helpful to you.
If the answer is helpful, please click "Yes" and kindly upvote it. If you have extra questions about this answer, please click "Comment."
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.