If the volume is encrypted with bitlocker and they don't have the password or recovery key (this triggered because you made a change to the firmware) then a reinstall is your only option.
You won't be able to recover any data from that drive since it's encrypted unless they have data on a second volume that is not protected with bitlocker.
*Edit* just saw the Microsoft account mention, they will need to go through the recovery process of that account password
https://support.microsoft.com/en-us/help/4026971microsoft-account-how-to-reset-your-password