If you are troubleshooting SharePoint search that does work in general, bot does'n work for a specific list or library
(i.e. Search box not working),
and you already checked list settings -> advanced -> Search.. Allow items...
and you already tried to reindex list and site,
it's time to ensure that default item view form is in place and works ok.
Of not - just create a new display item form and make it default.