Not all score collections or larger works have their song contents fully indexed in the library catalog. It's not that we don't have your song, but you might have to try more than one search strategy before you find it!
First, try searching SUMMON for the title of the song or aria
This will retrieve scores for which the title of your song or aria is typed into the contents notes.
Second, try searching SUMMON for the title of the larger work that contains your song or aria (opera, song cycle, musical, or oratorio)
This will retrieve scores that will contain your song or aria, though the catalog record may not have the song or aria title listed.
Third, for art songs, try searching the Classic Catalog for the composer's last name and the word "songs"
This will retrieve scores that are song collections by a single composer (i.e. 200 Songs for Voice and Piano by Schubert).