First, please shut down Nuendo. Then go to the Start menu, All Programs, Nuendo 6_64, User Settings Data Folder. In the top address bar, you should see “App Data (roaming) Steinberg” Click on Steinberg at the top. This will put you in the Steinberg data folder. Trash any Cubase or Nuendo folders that you see there. This will trash your key commands, preferences, sound card settings, etc. It also should help to clear up the problem you are having. Restart Nuendo. You will need to reset your audio outputs (Devices, Device Setup, VST Audio System).
Afterward go to File, New Project, and select the “Empty” template option. Go to Project, Add Track, Instrument, and load up Omnisphere, and tell me the result.