The round mark on the glass is the glue that held it on there. Go to an auto parts store and get a kit to glue the metal piece back to the glass. Get some masking tape and put a piece on the windshield where the bottom of the metal piece is supposed to be, also put a piece to mark the location of one side of the metal piece. Then take a razor blade and gently scrape the remaining glue from the glass so you can put the mirror back to where it's supposed to be. Remove the metal piece from the mirror, the mirror usually slides on the metal piece from the top and has a screw to keep it tight. Follow the directions on the package to clean the glass and the metal piece, then apply the glue and line the metal piece up with your tape marks. MAKE SURE YOU GLUE THE RIGHT SIDE OF THE METAL PIECE TO THE GLASS! Use another piece of tape to hold the metal piece in place while the glue dries, it will take longer than a minute. The package will say how long to wait before putting the mirror back on.