Hello,
Have TFTP server with latest SSG5 OS directly connected to lowest port on SSG5.
Reboot the device and when prompted 'Hit any key to run loader' enter any key and follow the below KB article:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB5519&smlogin=true&actp=search
And check if it works. If it does not work, try same procedure but instead of ScreenOS firmware, upgrade the bootloader as given in the link below:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB10976&actp=search
Regards,
Rushi