In the database, you can set the password for a Site Administrator user to NULL. This will allow you to log in to the Quality Center Site Administrator by just specifying the Quality Center Admin User Name. Once logged in, you can change the password to whatever you wish. This is assuming you're using Quality Center authentication rather than LDAP.
If needed, run this query first to provide you with the list of users that are Site Administrators:
SELECT * FROM td.USERS u
INNER JOIN td.USERS_ROLES ur
ON u.USER_ID = ur.UR_USER_ID
INNER JOIN td.ROLES r
ON UR_ROLE_ID = r.RT_ROLE_ID
WHERE RT_ROLE_ID = 10
Once you know which Site Administrator user you want to change, run this query to set their password to NULL (replacing `YOUR_QC_ADMIN_USER_NAME_HERE` with the correct user name):
SET USER_PASSWORD = ''
WHERE USER_NAME = 'YOUR_QC_ADMIN_USER_NAME_HERE'
Restart the HP Quality Center service.
Log into Site Admin using the user (leave password blank).