I know all too well what you're going through. Perhaps moreso because I've known the numbness but I've also known a complete emotional collapse upon losing a loved one.
When I was about fourteen, my grandmother passed away in her sleep, leaving my grandfather to live alone with us. I was raised by both of them, and spent a large portion of my early years in their presence. They loved me very much and I knew this, but her death stirred no emotion in me. I tried to cry but felt nothing, and I wondered if something was wrong with me.
My mother would ask if I was upset, and why I was not crying, and I used the fact my grandmother always said that we should not cry when she passes, as she's going to a better place. That we should rejoice instead. But that was just my way of justifying my complete lack of emotion. It was an odd thing; her passing, seeing the ambulance outside my home; her body in the casket. All very odd, but emotionally stirring? Not in the least.
I came to believe it was just that I did not feel emotionally tied to her. I appreciated all she'd done for me, but I never knew her. I never felt close to her. So this lack of emotion made complete sense, as does yours. It's no measure of your ability to feel or love. You just know deep down that this death is not something that will affect your life.
When my mother died, that was different. At one point, as she was laying there dead before me, I placed my hand on her head in some fashion as to bid a last farewell before the body was taken away. And I broke down completely and began to sob heavily. It was a moment of about five minutes, where I could not control my tears and I felt genuinely broken at the thought of her absence from my life.
Aside from the church service and funeral, where I cried quite a bit, the rest of the services were very calm and collected for me. The burial, the wake, during neither of these did I grow saddened. No tears were shed then. The human psyche has an astounding ability to adapt to life changes, and let it be known that within a day of her passing, my family and I had pulled together and were no longer crying about my mother's death but laughing, telling stories.
I suppose what I mean to say in all this, is that your reaction is normal given your relationship with your grandmother. There will be people in your lifetime whose deaths will bring you to weep. Others will pass without an ounce of lost sleep. This is life. This is death. They are not two separate things but different parts of one unified cycle whole.
I hope any portion of this has helped to bring you some peace of mind. I love you and I hope your family is handling everything well. You'll be in my thoughts this holiday season and beyond. Take care of yourself, please.