05 focus issues, please help

i have a 2005 focus, i drove the car to LA for a small trip to the auto show. when i went to leave and start my car the battery was dead. the car got jump started and i made it home. on the way home the car would run rought/lights would go dim.I got home shut off the car and tried to start it. it didnt start. so i figure it was the alt. i went and got a new battery (as the old one was origanal and needed a new one soon) put the battery in the car and the car still wouldnt start...so what could this be? even with a dead alt the car should still start. any help would be great.


thanks