Having old habits configuring Cisco gear you probably will fall on this too, if you type CTRL-Z on CLI at Juniper device you will suspend the process. So the question is how to take control again over that process?
After type CTRL-Z
root@R1> configure Entering configuration mode  root@R1# Suspended
After you try using again the config mode, you get the following message
root@% cli root@R1> configure Entering configuration mode Users currently editing the configuration: root terminal d0 (pid 4409) on since 2014-10-15 04:36:13 UTC 
Getting again the previous session from the suspended space
 root@R1# exit Exiting configuration mode root@R1> exit root@% jobs  + Suspended cli root@% fg 1 cli root@R1#