I had this problem with a 2001 Prius (which doesn't have the button below the steering wheel). Checked the batteries in each key and fuses etc., but couldn't find anything wrong. Then a day later it started working again. It must have been caused by radio interference. (A weather station I have that uses similar frequencies also stopped and started working at the same time.) I haven't found the culprit but the lesson to learn is to try moving your car if this happens.
