Hello,
If you do not have credentials, try to reset the password which reset firewall to factory default.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB4749&smlogin=true&actp=search
Then you can try to upgrade the ScreenOS & bootloader which is available on Juniper site.
http://www.juniper.net/support/downloads/?p=ssg5#sw
Regards,
Rushi