What Causes Severe Hair Loss?

Hello Doctor, Am 32years old female with 2 kids. My younger kid is around 1.5yrs old now. The problem which i have currently is severe hair loss mainly on the centre portion of the head. you can clearly see the hair thinning around in this area. I dont see hair thinning on the rest of my head... Am also using hair dye due cover up the grey hairs which i have (my hair started greying right after the birth of my 2nd baby majorly during the first 3 months of her birth and post that it stopped. i could see huge portions in the front of my head greyed out during the 1st three months of the birth of my 2nd kid) My queries : 1) the hair loss at the centre of my head - is this due to post partum issues? 2) is there any tonic/syrup or medicines which i can take for my hair regrowth/hair loss 3) is there any medicine which i could take for further stopping of grey hairs and for turning the grey hairs to black again. please help me. Regards Priya