The usual solution is to completely uninstall every Halion Sonic SE library by going to the Windows Control Panel and clicking Programs and Features.
After that, reinstall Cubase using the FULL Cubase installer found in your MySteinberg account. Make the installer reinstall every component.
It might also help to install Halion Sonic SE 3 and the latest update for it, which has smarter library detection and many bug fixes. Use the same email you used for your MySteinberg account to receive a download link.