How do i get rid of an unwated ad that is blocking my 'new mail' blank forms?

I am trying to send a new e-mail. I have a pop up advertisement that is entirely blocking the body of the letter (i.e. where should be writing my message) with absolutely no way of removing it. I have tried to right hand click to no avail, there is no cross in a box to help me. Please help, I am desperate. The ad is for stationery decoration.