How is retinoblastoma caused? What kind of mutation is it?

I can't find the answer to my question anywhere

I think I found what you are looking for.

Check out this link for the American Cancer Society, it has a section on causes of retinoblastoma:
http://www.cancer.org/docroot/CRI/conten...

It is caused by a mutation in the Rb gene. It's a loss of function mutation where the normal gene product, RB protein, fails to be produced. This protein is important in controlling the cell cycle.