The first thing to do (after panicking, obviously) is to check that you have really lost your contact lens, to make sure that it has not just slipped temporarily. It is very helpful to blink several times, and look straight up. Then, try squinting you eyes really hard. Focus on some far away object (or small text, if you are far-sighted) and wink each eye respectively. If you see a significant change, you contact lens has probably fallen out (by what method, one cannot imagine).
If you think, for some reason, that the contact is on the floor/ground then be sure to look around, and, of course, ask others around you to help.
If you can't find it, quickly get yourself to a mirror (but not too quickly, or you may either bump into something, or lose you contact). I find, most often that your contact is simply resting on your check, eyelash, chin, or shoulder.