My father has been experiencing nape pain for 4 months now. Taking NSAID's won't give relief. What would be the possible cause?
We really can't diagnose over the internet. However, anyone who has been experiencing chronic pain for a period of time should see a physician about that. It sounds like it's time for him to get to a doctor.
In the meantime, here are some pages that might help or be informative:
Cooking to Control Pain
10 Chronic Pain Myths
The Most Notorious Sources of Pain at Work and Their Solutions