At some moment index became bad and queries does not return any data.
For example, there are two tables - A (id, name) and B (id, name, a_id). B.a_id is foreign key to A. Both name columns in tables contains identical values for A.id = B.a_id. A.name column has unique constraint and additional index by it.
So, in some moment results for queries like [select id from A where name = 'petya'] became empty (row with 'petya' name exist in A).
But query [select a_id from B where name = 'petya'] returns A.id and [select * from A where id = <found id>] returns row.
This problem can be solved by index recreation only.