There are a few things you can try. Click on the "Home" icon in the Browse pane and then click on the Update Plug-ins button. Also, go to Options -> Locations -> VST Plug-ins and see what directory is listed under "Locations." Then you can go to your desktop and check to see if the plugins are actually in that directory.
If you're using Studio One Artist, you won't be able to change or view your plugin locations. In the "Artist" version, the VST locations are permanently set to these directories:
c:\Program Files\Common Files\ ( VST 2/VST3)
c:\Program Files(x86)\Common Files\ ( VST 2/VST3)
Another thing you can try is to go into Studio One's effects list, click on the 'wrench' icon, and check to see if any plugins are hidden. It will be grayed out and there won't be a dot next to it if it's hidden.
You can also try deleting the settings files for Studio One. This will reset the blacklist. Here are instructions:
1. Go to Help > Open Settings folder.
2. Copy the contents of the folder over to a folder on your desktop.
3. Close S1 and delete the contents of the settings folder with the exception of the user.license file and the "Extensions" folder.
4. Open S1 and go to Studio One > Options to reconfigure your settings if necessary.
5. Test to see if the problem is resolved. If it is not, please feel free to copy your settings files back over from the backup that you created in step 2.
*If you are missing presets or Sound Sets after doing this, simply double-click one of your Sound Sets under Documents > Studio One > Sound Sets.