A Soft Reset will power your device off and back on without deleting data.
To Perform a Soft Reset:
Press and hold the Volume Down and Power keys simultaneously until the device reboots. Typically this will take 11-12 seconds.
Release the keys when you see the LG bootup logo
Note: Depending on the state of your device you may see it perform a screenshot and/or a pop-up as shown below. This is normal.