Some members have reported that after using the clip View, and Unchecking a clip, the card has become hidden in the collection view.
You can tell because in the Upper left it reports 123/126 cards ( 3 hidden )
and also the Last page of your card collection will have a card Back with the number of hidden cards it.
To View all cards, type \w in the search box.
this will display all cards even the Hidden ones.
Then if you do a rebuild All, it should bring them back out of being Hidden.
Not sure what the actual cause is.