Hi,
Try logging in with admin username and password and see if that works.Otherwise you can reset the password.
Check link : http://www.cisco.com/c/en/us/support/docs/security/firesight-management-center/118631-technote-firesight-00.html
You also might need yo check the logs on command line and see whats the exact problem. YOu can view that under : /var/log/messages
Regards,
Aastha Bhardwaj
Rate if that helps!!!