I have seen this issue and it is always associated with an attached DV device that also is not functioning correctly with Pro, like a DV camera with no signal to its LCD from Pro. I have also seen the Program Monitor freeze a video frame while the timeline is playing.
Shutting down Pro with these issues seems to cause it’s GUI to go away but it is still running as a process.
Shutting down the device first seems to fix it.
When the external device fails to get a video signal from Pro, I go into playback settings and even though the Export setting is correct, I click the drop down menu and reselect the DV device and about 3 seconds later all is well…strange.