If your PC has BitLocker (HDD encryption), which is a standard function of Windows 10, enabled, you may be prompted to enter the BitLocker recovery key at an unexpected timing, such as when there is a problem booting the OS. In that case, you will not be able to use the computer until you enter the key.
If your PC uses Word or Excel downloaded and installed from Office 365, the recovery key for that PC may be stored on the Office 365 server. Check using the method in [Step 1] below.[Step 1]
Sign in to Office 365 with your YNU email account > "View Account" icon in the upper right > "device" > Select the problematic PC > "Show BitLocker Key" > "Show recovery key"
If you cannot verify the recovery key with this procedure, the IT service center basically does not know your recovery key. Check the following.
You can save your recovery key "in advance" to your Office 365 server in case something unexpected happens. If you cannot confirm the recovery key by the above procedure even after enabling Bitlocker, save the recovery key by following the procedure in [Step 2] below. After that, check if the recovery key is saved in [Step 1].[Step 2]
On that PC, "Windows Settings" > "System" > "More Information" > "BitLocker Settings" > "Recovery Key Backup" > "Save to Azure AD Account"