When the play button is pressed, the timeline does not play. Dragging the playhead will go through the preview, however, the play button will not play the video. Pressing play turns the play button to a pause button, but the playhead does not move.
This can be caused by certain audio devices set as your default playback device, such as wireless or Bluetooth headsets. Disconnecting those audio devices and restarting Camtasia can allow the play button to work. If this works you may be able to reconnect the audio device without further issues.