VST plugins empty in Cubase 10?

I don’t know why but when I click on ‘VST Instruments’, all my plugin players are listed by name but have no image and when I open them, they are empty. My Kontact player for instance. I tried going to STUDIO/VST PLUGINS and adding their folders, restarting cubase, but still nothing.
Can someone help me?

Could you explain what you are doing when you say you open them? Have you tried dragging one into the project area to create an instrument track? Did you go to the VSTi tab in the right zone and add a track or rack instrument?