If you completely breathe out, you will not be able to breathe because the diaphragm is weakest at that position. If you maintain air in your lungs you can breathe and speak. A person wouldn't know they actually couldn't breathe until they completely deflated their lungs. IE if you can say something you can breathe. It takes massive amount of weight spread widely across the chest to actually not be able to breathe. Now are we caught up.
Most of the time when people can't breathe, they are just having a panic attack, or are just panicking in general. Which is why when people have panick attacks, they say "I can't breathe" often.