When she went to the Montgomery County prosecutor in 2005 about the incident, the authorities reportedly didn't feel they had enough evidence to try Cosby.
The Associated Press does not typically identify people who say they are victims of sexual assault unless they grant permission, which Constand has done.
Bill Cosby leaves the Montgomery County Courthouse during his sexual assault trial, Friday, June 16, 2017, in Norristown, Pa.
Bill Cosby's chief accuser is thanking prosecutors for taking her case after a jury failed to agree on a verdict.
Lawyers for Andrea Constand issued a statement on her behalf Saturday. They had much of the key testimony from the first six days of the trial read back to them.
When Montgomery County District Attorney Kevin Steele announced that he plans to retry the case, there was a small gasp in the gallery.
He will remain free on one million dollar (£782,000) bail. Just before she lost consciousness, Constand testified, "I began to slur my words, and I also told Mr. Cosby that I had trouble seeing him, that I could see two of him".
Gianna Constand also testified that Cosby described the encounter through their phone conversations.
Comedian D.L. Hughley wrote on Twitter: "A jury deliberated longer in the #Cosby Case!"
Constand did not appear at the news conference.
Cosby's team declared victory and went on the attack.
Cosby used his power and fame to violate Constand, an employee of Temple University's basketball program, Assistant District Attorney Kristen Feden said in her opening statement.
Cosby's wife, Camille Cosby, slammed prosecutors for bringing the case to court.
Constand also noted that when she came to, she felt "humiliated" and "confused". Schaffer led the 2005 investigation into the allegations that Cosby had drugged and sexually assaulted Constand.
Shaffer said Cosby told him that he did not have sex with Constand at any point in time, and that they engaged in "petting" during the night in question.
Early Thursday the jury in the Norristown, Pennsylvania courtroom informed the court that they were unable to reach a unanimous agreement on any of the three charges the comedian faces.
By the time a mistrial was declared Saturday, the exhausted jurors had spent more than 50 hours in deliberations. Cosby admitted to giving the prescription sedative to multiple women with whom he wanted to have sex, and said that he didn't take the drug himself. Constand's case is the only one in which Cosby has been charged.
Some 60 women have come forward over the past couple years, accusing Cosby of drugging and sexually assaulting them, according to news outlets.
Cosby continued in her statement: "How do I describe many, but not all, general media?"
Prosecutors have four months to decide whether they want to retry Cosby or drop the charges.
The jury resumed deliberations Saturday over whether the 79-year-old TV star drugged and molested a woman in his Philadelphia-area home in 2004.
But the jurors clearly struggled with their verdict, telling the judge on Day 4 they were at impasse. All told, if found guilty, Cosby could spend the rest of his life in prison. They repeatedly asked a judge for a mistrial on Friday.
Judge Steven O'Neill refused, saying there's no precedent to halt the jury's work. Lawyer Brian McMonagle told jurors that while Cosby had been unfaithful to his wife, he didn't commit a crime. He maintains it was consensual.