According to various reports from Twitter users leaving the Home Run Derby at Citi Field, there is a fire somewhere on the 7 train line—initial reports indicate there is an investigation at the 103rd street stop—and it isn't running. The 7 is the only subway that services Citi Field. There are obviously buses and the Long Island Rail Road, but most people take the subway.
Newsday's Neil Best said the LIRR is doing what it can, but many folks are stuck waiting.
Update: Here's a picture from the scene: