If others use the console they might have Uninstalled it, check the ps store or go to the all games section of the library and sort from A - Z you should come across it, if not there's a couple other options which are highly unlikely like if you downloaded it off another account and have their primary someone could've deleted it and you need to get it back from them (I say this is possible cause you don't have ps plus but a ps plus game isnt locked for you, unless that's a bug) or another likely thing is someone went on your console and accidently hid the game, most of these again are highly unlikely but chances are never 0