Was it just the perdal or the whole 'crank arm'?

pedals seldom come off if they are only the weeest bit tight, but square taper crank interfaces are notorious for falling off if they aren't propely tightened during assembly (usually at the factory in Asia).

As an added bonus, if the crank arm fell off, it is very likely damaged from riding with is loose and will never again stay attached to the bike. Replacement crank arms are about twenty bucks.