When EX4650 device does not work properly or does not boot normally, performing a Junos recovery may resolve the issue. 

A recovery image is created automatically on the switches. If a previously-running switch is powered on and unable to boot using a Junos OS image, you can boot the switch using the recovery Junos OS image by selecting one of the saved images.

To recover the switch, in case of image corruption, perform the following steps:

1. Power cycle the switch.  You should see the following after the bios testing. 
2. After the bios testing, you will see the message 'Esc is pressed.  Go to boot options';  press Esc to get in the boot menu:     
3. The boot Menu is displayed, select Boot Manager:    
4. Select the HDD to boot from:   
5. You should now be in the GRUB menu, as shown below.  From the GRUB menu, select Juniper-Linux- Recovery.    
6. The menu for recovery will be shown.  From the menu, select the image you want to recover to (in the example, recovery image #2 was selected).
7. Next answer the following questions, (1) do you want to format the Junos data disk, (2) do you want to format Junos config disk, and (3) do you want to reimage linux host. 
   
   In the example, we chose (1) Y- format the Junos data disk, (2) N- to not format the Junos config disk, to preserve the config disk to use the old config, and (3) selected Y since you must reimage the host so it will match Junos:  
8. Last, confirm your choices, change it or cancel:  In the example option 1 was selected, 'Proceed with the recovery'.

For QFX10K and EX4300-MP, refer to KB33990 - Recovery installation for QFX10K and EX4300-MP. [juniper.net]

For QFX5100/EX4600, refer to KB34866- How to recover Junos on QFX5100/EX4600 [juniper.net]