Almost 4 Months Ago I Had A Hard Skin On

Almost 4 months ago I had a hard skin on my sole and it started hurting when something hit it. Then the colour of that place started changing it become dark and hard and its getting worst. I consult a doctor he prescribed me salicylic acid to apply on it. I used it regularly for a month thrice a day but nothing happened to this thing
what should I do now ?