Press F5 to open the Cubase MediaBay and click “Update Results” at the top. Take a look through the File Browser section of the MediaBay and check if everything is in order.
If your presets still don’t show up, you can try forcing a full MediaBay rescan:
Missing Loops & Samples - #2 by Romantique_Tp
Please make sure to backup your mediabay3.db file.