If I understand you right, the Cover Art for TV program Series isn't showing, but the Cover Art for Seasons is showing, and I assume for Episodes as well.
First, since it used to work, check if the Series Cover Arts files are still there on disk. They probably are. If you have left Cover Art in the default location you will find the Series Cover Art in;
"C:\Users\[UserID]\AppData\Roaming\J River\Media Center 21\Cover Art\Series".
If you have change the location that Cover Art is stored, the setting to check is at "Options>File Location>Cover Art>Folder".
If the image files are still in the correct location, then it is likely that the Thumbnails for you Cover Art have been corrupted or messed up, and you may need to rebuild them. That may take some time, but if you want to do that just go to;
"C:\Users\htpc\AppData\Roaming\J River\Media Center 21\Thumbnails"
and delete everything in that directory. Then select all files in your library, right click on them and select "Cover Art>Rebuild Thumbnail". The Thumbnails will also be rebuilt automatically once MC notices they are missing, but that may take some time.
Alternately, you could right click on a Series icon in Standard View, and select "Cover Art>Quick Find In File / Cover Art Directory", which swill find the image files for the Episode Cover Art, and may find the images for the Series and Season, and update the Thumbnails for all. Maybe. Certainly the Episode art is updated this way.
Alternately, you could right click on a Series icon in Standard View, and select "Cover Art>Get From Internet" which will download new Cover Art for the Series and Season, which will also update the Thumbnails.