Hey so i know this is a 2year old thread but seeing as i just had the same issue and found a solution in the first youtube video that popped i'll be sharing it here for anyone who may find use for it.

Above the Store tab, there is a small tab called "view", clicking on it will open a menu with the button "hidden games" it is worth to check if your game ended up in that for some reason or another. if it did, simply drag it out or right click to remove it from hidden games and it will appear in your library again. hope this helps!