...have no idea what happened (i don't use screensets) pressing "1" did not help.
How to get back to the original Default screen layout?
 

To show the Start Screen
By default, the Start Screen appears automatically when you open SONAR, and when no projects are open.
To open the Start Screen any other time, use the File > Start Screen command.
 
To prevent the display of the Start Screen
1.
Use the Edit > Preferences command to open the Preferences dialog box, then click File - Advanced.

2.
Clear the Show Start Screen on Launch check box.

3.
Click OK to close the dialog box and accept your changes.

The Start Screen isn't displayed the next time you launch SONAR

You can also choose File > New to open a screen that's like the old one, but IMO better.
 
Don't count the start screen out yet, though. Several improvements are in the works.