This is the cabling pin out for the RJ45 to DB9 cable needed for the console cable. Juniper devices ship with a 9pin adapter with the rj45 on the other end. They are the same pin out as the blue cisco cables if you have any of those around.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB4066
Once connected the output during the boot process should give some information on the nature of the problem.