Or she just became old? Hollywood has limited leading roles for females, and even less for women between 35 and 55. Look at Julia Roberts after Erin and Ocean's, Uma Thurman after Kill Bill, even Charlize Theron, Halle Berry and Nicole Kidman seem to have become second-tier. In that range I can only think of Jennifer Aniston and Sandra Bullock in the category of people that still sell movies as the lead. They are too old to compete with the new starlets that attract a teenage audience (Stewart, Fox, Emma Watson, ...) or that are convincing enough in romantic rules (Reese Witherspoon, Zoeey Deschanel, ...) and too young to compete for the more mature female roles. That category is dominated by people like Judi Dench, Diane Keaton, Glenn Close, Helen Mirren or Meryl Streep.
Most of the female leads that have disappeared, have become famous because of their romantic roles, something you can also see with male leads. Action stars (Pitt, Cruise, Damon) and character actors (Depp) have a longer life as an actor, but there are less roles like that for females.
