Step-by-Step: https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/troubleshooting-instructions-for-cubase-nuendo-and-sequel-on-windows/kb_back/2020.html
Including Cubase Safe Mode: https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/safe-start-mode-resetting-the-cubase-preferences.html