Closing Excel with the X button

In my Excel program, my X button in the top right corner is there, but it is gray and will not close the program. I have to goto file/exit. I still can minimize and maximize with the ajoining buttons. On some other's Excel, I have seen the whole barmissing that has the X, min, and max buttons. I figured this is a setting that can be change, and I would like to know how?