helllp! it keeps turning back :(

ahhh, once a color I loved for me and now is a disaster...I have colored my hair, removed the color, had a pro attempt a deep brown that turned black, removed again and colored light brown, but it turned out black again? My hair is oddly enough, very healthy and rarely even needs a cut. What should I do to return to my natural color medium/light brown? I used a ion light brown N5.... and it is ba-lack! please help...for you are probably the most beautiful!!!!
xox