Press the "door open" button. Sometimes, this button can just get jammed, and if you press it, it'll open the elevator right up. You may be laughing, but you'd be surprised by how many people call for help to escape a stranded elevator only to find that they just have to press the "door open" button again.
You can also try the "door close" button, which may have gotten jammed as well.
You can also try pressing the button of a floor below where the elevator is currently resting.