Could you be pregnant? LOL I only ask because 140 is exactly where I was stuck. I ended up reducing my cals to between 1300 and 1400 and upping my exercise to nearly 90 min. per day to break the plateau. I managed to get down to 137 in one month...before finding out I'd been pregnant for 2.
It could be that you need to decrease your cals, up your exercise or both. Or it could be that you're not eating enough cals compared to the amount of exercise you do. Try playing around witih the numbers. Increase the exercise and decrease the cals (but try and stay above 1300) for a couple of weeks. If you don't notice even a slight drop, try increasing your cals a bit for a couple of weeks and see if that has an effect.