This symptom can be attributed to many different factors. Often times a simple reset of your device will clear up issues that are causing your device to freeze or become unresponsive.
This device has a non-removable battery. Do not attempt to replace the battery yourself or you may damage the battery, which could cause overheating, fire, and injury. The lithium-ion battery in your device should be serviced or recycled by LG Electronics or an authorized service provider.
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.