When LDAP server is down, no users can log into QC/ALM and this issue should be solved by modifying the DB backend.
Below are the steps to switch the authentication mode:
1. Access qcsiteadmin_db schema;
2. Select the PARAMS table;
3. Change the PARAM_VALUE from ''LDAP'' to ''QualityCenter'' for the PARAM_NAME ''AUTHENTICATION'';
4. Restart the QC/ALM service or reboot the server machine.
Note: If the database schema cannot be accessed, please consult with the responsible DB Administrator.

The following SQL is used to reset the authentication mode:
UPDATE PARAMS
SET PARAM_VALUE='QualityCenter'
WHERE PARAM_VALUE='Ldap';

You will be able to log back into Site Admin with the Site Admin user account that was created when the software was originally installed.

NOTE: The Site Admin user account is uniquely defined during the original installation. If this user account is not known, I suggest you review any custom install documentation that was created showing what was input during the installation.

Once you have gained access to the ALM Site Admin, you can update the LDAP settings with the new password and switch the authentication mode back to LDAP. Keep the browser open to that instance of Site Admin, and confirm your LDAP settings on another machine.