You have lost access to the router through other means and you need a way to log in.
Log in to the router using the console port:
aviva@server1% telnet router1-con Trying 172.19.121.19… Connected to router1-con.mycompany.com. Escape character is '^]'. router1 (ttyd0) login:
The console port on JUNOS routers is enabled by default. If you ever lose access to the router through normal login means, you can log in using the console port. While it is possible to disable the console port, it is really not recommended.
Use of the console port is not really required other than when you are initially installing the router. However, many people use the console port as the access of last resort. You can set up a terminal server with console connections to a number of devices in the event that the network fails. Also, if someone accidentally misconfigures the router and locks themselves out, or if routing has failed, you can still get into the router remotely using the console port.
Access over the console port is the only method that allows you to remain connected to the router during a reboot. A reboot logs you out of the session on the router, but you will still be connected and will be able to halt the reboot or watch the messages as the router reboots.
Router Configuration and File Management
Basic Router Security and Access Control
Routing Policy and Firewall Filters