We can only advise this for Windows PCs
Be aware this will remove any and all Steinberg products, not just Cubase!
In Windows, go to 'add or remove programs' in the start menu (just start typing it, it will show up). Scroll down and uninstall all 'Steinberg' entries that are there.
Next use file explorer to go to the following directories. In these directories you will find more 'Steinberg Folders'. Just delete any folder that has 'Steinberg' in the name. Do not delete anything else. Please note, some of these folders are hidden folders. To show hidden folders follow the instructions here https://support.microsoft.com/en-gb/help/4028316/windows-view-hidden-files-and-folders-in-windows-10
Potential Steinberg folder locations:
C:\Users\(your user name)\AppData\Local
C:\Users\(your user name)\AppData\Roaming
C:\Users\(your user name)\Documents
After this is done, Cubase and any other Steinberg related products have been removed from your computer. To reinstall, download the 'Stienberg Download Assistant' from HERE. Find the version of Cubase you are using and download the 'Full Installer' (it should be over 20GB). Once downloaded, install it and you should have a fully working fresh version of Cubase installed.