First, please shut down Cubase.
Then go to the Start menu, All Programs,
Cubase 5, Cubase 5 Application Data Folder. Press the Up Folder Button (green arrow pointing up in the top left corner). This will put you in the Steinberg app data folder. Trash all of the folders that you see in the application data folder. This will trash your key commands, preferences, sound card settings, etc. It also should clear any problems you are having. Restart Cubase. You will need to reset your audio outputs (Devices, Device Setup, VST Audio System).