Sometime Juniper devices may not boot JunOS properly. The image might be corrupted. So you need to re-install the JunOS of the EX or SRX device from the loader prompt. It is always good idea to have a USB snapshot of the JunOS device. But if you don’t have backup then you have to re install JunOS from loader prompt using USB media.
Install JunOS From Loader
Step 1. Download the required JunOS and copy to USB flash drive. If the USB size is less than 2 GB then the file format must be FAT. Similarly, if the USB is greater than or equal to 4 GB, use FAT32 file system format.
Step 2. Now insert the flash into USB port of the Juniper device.
Step 3. Then, reboot the device. When JunOS boots up,press SPACE BAR on your keyboard to get into loader prompt. At the boot process hit SPACE BAR key when the following message is displayed,Press Space to abort autoboot Do nothing. A little while later, you will see:
Hit [Enter] to boot immediately, or space bar for command
Press the space bar. You will be at loader mode; the prompt should
be loader>. If the prompt is > , type >boot to make it loader>.
Step 4. After hitting the SPACE BAR, you will notice the loader prompt. Now type the following command:loader> install file:///jinstall-ex-2200-11.4R11.4-domestic-signed.tgz
Wait for a while, the installation file will be copied to the device and and start the installation process. The device will reboot after the installation is complete.