If your mirror has fallen off the windscreen, unless it is the type that is screwed on or fixed to the car, you will need one of these to fix it back on.
This will not work on very heavy electric mirrors.
The Best way to fix it back on is to clean the glass with some type of alcohol methylated spirit Brandy, whiskey that type thing, not white spirit.
Also make sure the mirror is cleaned also the same way.
Then peel back one side of the pad and stick it on the mirror.
When the weather is cold outside any sticky pad will not work properly and may fall off.
So the best thing is to get a hair a dryer if you can. Warm up your windscreen and the interior mirror before you peel back the sticky pad and place on the glass.
If you cannot use a hair dryer just turn on the car heater to do the same thing.