I dyed my hair from brown to platinum if i dyed it black from a box dye will my hair break off?

i have naturally dark brown hair and i had dyed it platinum blonde sometime at the end of summer (2010) the person who dyed my hair used a 30 volume in bleach 2 times and used a toner i think 2 times as well, but i had realized that my hair grows about an inch every month and i cannot afford to have to dye my roots every 2 months, so now i want to dye it black with a box dye (today's date is january 8 2011) but im afraid that this will make my hair break off. what should i do? do you know if my hair will break off or not ? :(