My testicle is hard , however the blood test afp came back negative , what can cause he testicle to be hard ?

Here are some ...: A palpable hard testicular mass as visualized & confirmed by ultrasound should be considered as testicular tumor, most of times, as malignant disregarding (+) or (-) in AFP or Beta-HCG, which is merely suggesting presence of seminomatous or non-seminomatous component. Oftentimes, professional judgment plays an important role to decide how to deal with a hard testis, usually via inguinal incision.

Tumor: Since you had AFP checked,I assume you are seeing a doctor.AFP is found in blood of a patient with non-seminomatous tumors but never in a patient with a seminoma(which is a testicular tumor). It is present in about 75% of patients with teratocarcinoma,embryonal cell cancer and yolk cell cancer. There is no space for more details but PLEASE MAKE SURE YOU ARE SEEING A UROLOGIST.Best of luck.