S/D is investigating an issue where the client appears to freeze when switching between applications on Windows 10 Anniversary edition. Minimizing and restoring the client by clicking on the task bar icon twice unlocks the client until the next occurrence.
Another Windows 10 oddity is that the Horizon icon may display in the task bar but it will not open when you click on the icon. In this case, hover the mouse over the icon and a small window should display with the heading showing your location name. Right click on that and a small grey window should open with the option to "maximize" click on that and Horizon should open properly.
Updated 3/7/18 by Chris Magnusson