Is it possible to re grow hairs after head injury on that place?

It really depends on the depth of the injury and the mechanism of traumatic injury. Generally speaking, the deeper the injury the more likely there is to be permanent hair loss as there is a greater likelihood that follicular stem cells are affected. See a physician if concerned. 

It depends on the severity of the injury. Do you have pictures? If you have a scar where hair hasn't grown back within a few months, it's likely the only way that you'll be able to get back to a proper looking hair pattern would be through a hair transplant.

If you had a traumatic hair loss on your scalp resulting in a laceration and then healing, many times the healing occurs and there will be no hair in the scar, Hair transplants can solve this problem