In order to retrieve information from external sources, you will need to provide some basic details, such as the ARTIST name, the track TITLE and the ALBUM name.
Without such information - i.e. if a song is completely "unknown" - it is not possible to search for it on online sources such as Discogs or MusicBrainz.
The only way to search for an "unknown" song is to use AcoustId. This is an audio fingerprint created for the audio part of a track. This fingerprint can then be searched against a huge online database. If a match is found, the existing metadata is returned.
MusicBrainz Picard, for example, provides this kind of audio fingerprint creation and online search.