I feel that I've read about this problem here, but didn't find it using google (wrong search words, no doubt) ---
 
Sonar Platinum K version, windows 7; no changes to system or drivers
 
a saved project appears to load fine, but immediately shows as changed (has an asterisk beside the project name).    Does not completely refresh all windows and will not take any mouse or keyboard input.      
 
According to windows 7 Task Manager, Sonar is "waiting for input".   No other programs running, low but not zero CPU usage -- SONARPLT.exe using about 8 percent, higher than normal but low enough to easily start other programs.   No network.
 
If I end the task with task manager, and restart Sonar, I can load other projects including older versions of the same project. 
 
Any ideas how to find out what Sonar is waiting for and how to get to the point of providing input? 

Make sure that there are no hidden windows dialogs waiting for input. 

There may be some, but I cannot get to them in any way.     I've tried mouse click everywhere blindly, Alt-Tab, Ctl-Tab, all keyboard keystrokes, Scroll Lock.     And I don't know what dialog would be present.  
 
But as typing this I had an idea --- after yet another render midi oddity, I disabled some of the audio outputs last session of work.       I imagine Sonar is trying to ask me if I want to re-enable them, only it is doing so in a hidden and impossible to access way. 

Using Safe Mode, Shift + Click in the recent projects dropdown, gets the project file open at least (I clicked NO to every question).
 
 

On Windows 10 (Windows Key + Tab) will present all open windows.
 
For Windows 7 try this  (Ctrl+Windows logo key +Tab)
 
Edit to add. If you have dual monitors move the main window to the other monitor. This should reveal any hidden windows.

Ctrl+Windows+Tab shows only one task, Platinum, plus the desktop, so no help.
 
Moving the main window to another monitor (before opening the problem project) does not help; it can't be moved once Platinum freezes.
 
I tried moving the main window to each of my three monitors, and making it very small -- none of these tries helped. 
 
I have moved on by opening in Safe Mode, saving under a new name, and then opening that project.   Oddly, the new project has grayed-out instances of Kontakt, and no sound, but when I delete the grayed-out Kontakt and re-insert Synth and reload Synth, the project works again.   

If this happens again try hitting enter.

That was #1 in my list of attempted fixes.  Nope.