Here is Howto about the recovering procedure of a lost root password in JunOS. You need to reboot!
- Power off the router by pressing the power button on the front panel.
- Turn off the power to the management device, such as a PC or laptop computer, that you want to use to access the CLI.
- Plug one end of the Ethernet rollover cable supplied with the router into the RJ-45–to–DB-9 serial port adapter supplied with the router.
- Plug the RJ-45–to–DB-9 serial port adapter into the serial port on the management device.
- Connect the other end of the Ethernet rollover cable to the console port on the router.
- Turn on the power to the management device.
- On the management device, start your asynchronous terminal emulation application (such as Microsoft Windows Hyperterminal) and select the appropriate COM port to use (for example, COM1).
- Configure the port settings as follows:
- Bits per second: 9600
- Data bits: 8
- Parity: None
- Stop bits: 1
- Flow control: None
- Power on the router by pressing the power button on the front panel. Verify that the POWER LED on the front panel turns green.The terminal emulation screen on your management device displays the router’s boot sequence.
- When the following prompt appears, press the Spacebar to access the router’s bootstrap loader command prompt:
Hit [Enter] to boot immediately, or space bar for command prompt. Booting [kernel] in 9 seconds...
- At the following prompt, enter boot -s to start up the system in single-user mode.
ok boot -s
- At the following prompt, enter recovery to start the root password recovery procedure.
Enter full pathname of shell or 'recovery' for root password recovery or RETURN for /bin/sh: recovery
- Enter configuration mode in the CLI.
- Set the root password. For example:
user@host# set system root-authentication plain-text-password
- At the following prompt, enter the new root password. For example:
New password: juniper1 Retype new password:
- At the second prompt, reenter the new root password.
- After you have finished configuring the password, commit the configuration.
root@host# commit commit complete
- Exit configuration mode in the CLI.
- Exit operational mode in the CLI.
- At the prompt, enter y to reboot the router.
Reboot the system? [y/n] y
Mostly copied from here.