Hi, thank you for reaching out. I’m happy to help.
You will need the correct BitLocker key from the correct Microsoft account to unlock BitLocker. Since your father’s account, which appears to be the account where the BitLocker key is stored, shows the error message ‘too many attempts,’ and based on your description, it seems the account holder has already tried all the basic troubleshooting. The account recovery form is the last resort.
He will need to fill out this account recovery form to regain access to his account: https://account.live.com/acsr
Microsoft takes account security seriously, so the recovery process is fully automated to prevent unauthorized access. Complete the form using a device and location he used before, and make sure to provide an accessible email address.
Microsoft typically reviews recovery requests within 24 hours. If his request is successful, they’ll send instructions to help him recover his account.
Help with the Microsoft account recovery form: https://support.microsoft.com/account-billing/help-with-the-microsoft-account-recovery-form-b19c02d1-a782-dee6-93c3-dc8113b20c42
He can submit the form up to twice within a 24-hour period, so feel free to include any new details he remember each time until he recover his account.