Here is Steve (in another thread) explaining how to do it
_(Since you did not include any system info this is the long version)
First open the Preferences dialog in Cubase and click “Store” to save a preset to back up your user prefs, and thern quit Cubase.
Now rename the Default.xml (I do xDefault.xml).
(On Mac it’s located at /Users//Library/Preferences/Cubase 7.5/
Open the “Run” command prompt.
Windows Vista, Windows 7, Windows 8: Press the Windows logo key + R simultaneously.
Windows XP: Select ‘Run’ from the Start menu.
In the command line of the ‘Run’ window, enter this path: %appdata%/Steinberg
Locate the folder which is named the same as your Cubase/Nuendo version.
In that folder is the Defaults.xml file, delete it or rename it to force Cubase to reset to defaults.
Now relaunch Cubase, the Channel Edit window should now behave nicely again.
Next, open the preferences dialog again and load the Preference Preset you previously created. Your preferences will be restored, with some minor exceptions, and you should be back in business._