I am unable to open my mouth after bone grafting after tooth extraction because of pain.. what to do is it normal ?

Somewhat: Your difficulty in opening your mouth is more likely due to the local injections given and possible spasm of your muscles from those injections and swelling. Take some otc pain medication such as advil, Motrin or aleve, (naproxen) follow the instructions given by your dentist and if you have any concerns, call your dentist for advice.

Injection: Sometimes soreness at an injection site can cause muscle cramping ("trismus"). Go back to the dentist who did the graft for an evaluation. If something is wrong with the graft, quick action can prevent the loss of the graft.

Trismus: Inflammation due to infection may cause swelling in the posterior area, which restricts the movement of the TMJ joint (tmj disc specifically) and spasm of the muscle of mastication. This leads to trismus, or inability to open the mouth or limited mouth opening due to the spasm of the muscle of mastication. Corticosteroid (dexamethasone or Medrol (methylprednisolone) dosepack) and antibiotics should solve the problem.