I'm going to make an assumption. You have the 3.5SE! Your mirror folds in on the occasion that something pushes it (hitting something, pedestrian in parking lot, etc). Walk to your mirror and see if it's in the right position, if not, move it back!
LOL...I had the same experience when I first got the car and quickly found out that someone/something pushed it in. A simple pushing it back to where it belong solved the problem. The difference, I didn't post the problem..