This usually happens on hot summer day, I will get a call from a customer that their rearview mirror has fallen off or has become unglued. If the mirror has just came unglued your in luck, it can be glued back in place. If it has taken out a large piece of glass it will have to be replaced.
1. Go to your local auto parts store they sell kits to glue your mirror back in place.
2. To reattach go on the outside of the windshield and take a magic marker and outline around the remaining adhesive, this will mark the spot when you re-attach the mirror.
3. Take a straight edge razor blade and clean the remaining glue from the inside of the windshield.
4. Separate the V shaped piece of metal ( mounting tab ) from the back of the mirror holding structure this can be done with a small straight end screw driver, it should slide right out. Note: sometimes there is a small set screw that need to be removed also.
5. Clean the old glue off the windshield mounting tab using alcohol or rough it up lightly with emory cloth or sand paper.
6. Open up the rearview mirror mounting kit and follow the instructions.
7. Make sure you mount the mounting tab in the right position not upside down, wide part of the tab should face down refer to where you previously marked the outside of windshield.
8. When you glue the tab back to the windshield use a piece of tape to hold it to the glass giving it time to cure.
9. After the glue has set up slide the mirror back into place gently and replace set screw if needed.