When I turn on my Mac, the screen goes white with the flashing question mark. I have tried pressing down all the reccomended buttons but nothing has worked. Suggestions?

Our fairly old Mac has been slowing down over the years but it has never been a problem. Recently, when I start it up I get the blank screen and eventually the flashing question mark. I have tried pressing down shift as well as P R Command Option and Command R yet nothing has worked. Suggestions please, it has all of my pictures and documents on it!!!