Judith Miller was imprisoned for 85 days for refusing to identify a journalistic source she used that had supposedly leaked classified CIA information. This was a branch of the Plame Affair.
The Plame Affair started on July 6, 2003, when an op-ed piece was released for the New York Times that had been written by a former U.S. diplomat named Joseph Wilson. Joseph Wilson was the husband of a woman named Valerie Plame, who was an undercover agent that likely gave Wilson the information that he used in the article.
The article that Wilson had published contained supposedly "classified" information that had not been released to the public about the intentions behind the Bush Administration's decision to go to war with Iraq.
Later on that month, it was confirmed via journalist Robert Novak that Valerie Plame's portfolio of classified knowledge contained all of the information that Wilson put forward in his article.
This event sparked a large scale investigation in December of 2003 that was being led by prosecutor Patrick Fitzgerald. This investigation was called to support the 1982 law that made it illegal to reveal information about a covert agent to anyone to anyone not authorized to receive the information otherwise.
After interrogating the president, vice president, several journalists, and connected individuals, Fitzgerald settled on a woman named Judith Miller. Miller had spoken to an individual that was a liaison for Wilson and Plame shortly after the op-ed piece was published, and the CIA believed that she had more information about the case than she was willing to tell.
So, she was thrown in jail, unconstitutionally.
Exactly 85 days after she was thrown in jail, a man named Scott Libby, who was the liaison between Wilson and Plame, confessed his guilt, releasing Miller and commuting her sentences. Libby's sentence was later commuted by President George W. Bush.