Hair loss isn't caused by a lack of blood flow, and doing a handstand only causes congestion in the face and scalp. It does not make your hair grow.
With diffuse hair loss, the hair cycle is sometimes shortened.
There can be many causal factors: genetics, nutrition and deficiency, hormones, medicines or toxins, but there is no vascular factor and no lack of blood flow. Some effective treatments like minoxidil can induce vasodilation to provide the nutrients necessary for hair growth.