The first thing to do would be to see if the presets are missing from both the browser, not listing there under Open Air, and from the plugin UI, not listing there either. If not, go to the browser Home tab and click the button Re-Index Presets down near the bottom to see if they show up after that.
If they still do not show up, check Options > Locations > Sound Sets to be sure that the folder where your soundsets are located is listed there. If they still don't show up, refresh Studio One's application settings which will also clear out any files related to listings perhaps being cached or stored to see if that fixes it...
Go to Help>Open Settings folder.
Copy the contents of the folder over to a folder on your desktop.
Close S1 and delete the contents of the settings folder with the exception of the user.license file as well as the "Extensions" folder.
Open S1 and go to Studio One>Options (Preferences on a Mac) to reconfigure your settings.
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.
If after all of that if the same situation remains, please create a support ticket so that one of the support representatives can drill down into the issue and assist you.