Can mean all sorts of things. Are you saying that in the middle of whatever the phone simply stops responding?
Are you using a battery saver app? If so, get rid of it.
Do you have background data turned off? If so, turn it back on.
Using a notification manager such as Light Flow or Light Manager? Get rid of it temporarily.
Location services turned off? Turn it back on.
More than likely, you have a background process which is trying to do something but getting hung up. All sorts of apps try to "call home" and/or get the device's location on a regular basis, some of them don't like it when they can't accomplish the goal.