If you know the name of the song but don't know the artist or lyrics you can look it up online and listen to it. Similarly, if you know the artist's name or the lyrics you can also look it up and listen to it. If, however, one knows what the song sounds like...has the melody running through their heads but doesn't know the artist, title, or lyrics they're kinda screwed. At this point you're pretty much reduced to humming the song (and boy you'd better be able to carry a tune) for friends and asking them 'do you know this song'?. Know what I'm talking about?
I had a song with an iconic sax riff running through my head for days (much later found out it was a Gerry Rafferty tune) and no one could tell me who or what song it was. Most frustrating. There should be an app for that.