Multiple sources told celebrity news website TMZ.com that she went into full cardiac arrest and despite multiple attempts, could not be revived.
The Los Angeles City Fire Department told TMZ that emergency services were called to the house she shares with her screenwriter husband Simon Monjack in Los Angeles at 8am local time.
According to reports, her mother Sharon Murphy discovered her daughter unconscious in the shower.
Paramedics determined that Brittany was in full cardiac arrest and immediately administered CPR.
They continued mouth-to-mouth as they sped to Cedars-Sinai Medical Center, several miles away from her home, but Brittany was pronounced dead on arrival.
Monjack was not thought to have been at home at the time.
Murphy married screenwriter Monjack in a private Jewish ceremony in May 2007.
Celebrities began paying tribute to her on their Twitter pages today as soon as the news broke including rocker Fred Durst, who wrote: 'RIP Brittney Murphy. What a sweet human being. My prayers are with her friends and family.'
Reality TV star Khloe Kardashian wrote: 'OMG!!!! This is horrible news when anyone passes. I'm so sorry for her family. Send prayers out please.'
Singer Hilary Duff wrote: 'RIP Brittany Murphy forever in our hearts.'
Speculation immediately began surrounding the cause of Brittany's sudden death from cardiac arrest, the same cause of Michael Jackson's death in June.
Police are still investigating the singer's apparent dependency on prescription drugs.
Even celebrity gossip blogger Perez Hilton hinted on his website there might have been some other cause behind Brittany's sudden death today.
He wrote: 'Absolutely devastating. Especially because this comes as no surprise! We, and those who knew Brittany personally, saw this coming. That does not make this any less horrible.
'Our thoughts are with her family and friends during this extremely difficult time.'
'My worst vice is caffeine. [The rumor is] really pretty darn far-fetched -- it couldn't be further from the truth. I have known people over the course of my life that have bad problems with drugs, and it's something really serious.'
Brittany, an only child, was born in November 1977 in Atlanta, Georgia, but was raised in New Jersey.
She shot to fame after her breakthrough appearance in the film Clueless in 1995.
She played a mentally-disturbed young woman named Elisabeth alongside Michael Douglas in psychological thriller Don't Say A Word in 2001.
She also appeared with Kirsten Dunst in the black comedy Drop Dead Gorgeous in 1999 and Just Married in 2003 with Ashton Kutcher [Kabbalist; remember she married her current husband in a Jewish, probably kabbalistic wedding], who she dated for a year.
'See you on the other side kid.'
Her last big movie was Sin City in 2005.
In June 2006, she sang on Paul Oakenfold's hit single 'Faster Kill Pussycat' which got to number seven on the UK singles charts.
She dabbled in music again that same year when she covered Queen's Somebody To Love and Earth, Wind & Fire's Boogie Wonderland for the animated film Happy Feet, for which she also provided the voice for the character Gloria.