Open Spotify
Go to the album, playlist, or song list where the track is hidden
Find the hidden song
On mobile, tap the three dots next to the song
On desktop, right-click the song or click the three dots
Select Unhide this song or Hide from this playlist to toggle it back on
If it is in a playlist, make sure the playlist is not filtered or sorted in a way that hides it
If it is in your library, check that the song is not removed from the playlist or liked songs
Refresh Spotify or restart the app if the song does not reappear immediately