I am usually using my phone to debug my Android code. But sometimes you unplug your phone to use it and forget to plug it back in again. When you execute your code it will start the emulator (if you previously defined a device) but since it is taking some time to start the qemu Android image you have time to close it again and plug your phone back in but that leaves the Logcat (Android debug output console) empty.
In that case make sure you have the devices tab (Show view -> Android -> Device) visible and switch to the phone device to activate the Logcat output. In some coses you might have to start the emulator again and switch back and forth.