Hello,
First, please shut down Cubase 5.
Then go to the Start menu, All Programs, Cubase 5 and go inside the Cubase Application 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 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 up the problem that you are having.
Restart Cubase.
You will need to reset your audio outputs (Devices, Device Setup, VST Audio System).
Next open Cubase 5 and open up the Mediabay and right click on the VST sound folder and click "rescan".
Also inside Cubase 5 go to Help and About Cubase and tell me which version and build number it says you are running there.
Thank you.