Returning to Natural Hair Color After Dyeing It?

My natural hair colour is blond. I put temporary brown dye in my hair that was meant to last 30 washes. That was in July 2015. My hair is now a dark blond and my regrowth/ roots are dark brown. I want to go back to my natural hair. Can anyone tell me why my regrowth is now brown? How do I go back to my natural colour?