This first happened on a system with a 750 Ti.  The GeForce app displayed fine on my single monitor.  I added a second monitor for a test and then removed it.  Now when I try to open the GeForce app I get the startup splash screen and an icon in the task bar but no app window.  If I hover the mouse over the icon it shows a miniature app screen but clicking on it doesn't make it display.  I thought it might have gotten mixed up when I removed the second screen and it was "displaying" it inside the card on a ghost screen on the removed monitor.  I have another system with a 960 and the app was working fine until I added a second monitor.  The app now has the same behavior even with both screens attached.  Is this a common problem?  I'm running Win7.