I want to dye my already dyed dark brown hair light brown/dark blonde – is it possible and how?

In June I dyed my hair black and slowly its been coming out over the summer etc and was a medium/dark brown, i wanted to make it lighter so i bought ash light brown but nothing happened apart from my roots going lighter because it was dye free i assume. My natural colour is medium brown, that goes very light in summer. What can I do? Is there any point in me even going to the hairdresser if I dyed my hair a few days ago???