Dell XPS 17 9700

I have a three-year old XPS 17 9700 laptop that will not shut down with the regular "shut down" command.  Everything will appear normal, and the screen will turn dark as it should, but if I listen carefully with my ear on the keyboard, the fan will still be running and the laptop will not start the next morning.  The way I can get around it is to hold the on/off switch down for about 15/20 seconds, and then the fan will shut down and everything will be normal.  

Has anyone had this problem before?