Nitro Pro 11 (11.0.1.16) on a MS Surface Pro with Windows 10 was working fine for over two years but just now it won't open a window for any pdf file.  The icon is there, in the task bar, the file name is there, but the window won't open so I can see the file or work on it.  Any thoughts about how to remedy this so I can get to my pdf files with Nitro Pro?