I bought a song with my iPhone and its not downloading?

I wanted to buy another song from an album and as it was downloading I accidentally tapped it. Now it won't download and it's showing up in the list of songs from the album but its highlighted in gray.

Go back into iTunes and find the song and click download.

Ok, first go to the "More" tab at the bottom right side of the screen, then go to "Downloads" and try to redownload the song… Good luck.