Try taking the key out of your pocket. A button may be pressed while in your pocket. Others have reported opening and closing the driver door twice may work. Also, try using the key in the key fob to open the driver door. Another possibility is the battery is getting low on the key fob (see below). Some have reported disconnecting the battery fixes the problem.
Original battery lasted about 3 years from the time car was purchased. The original batteries were some months older. Battery is a 3 volt CR2032. To replace the battery, remove the key from the key fob and use a small screw driver to pry open the case on the end where the key meets the case. The + side of the battery goes down. Snap the case back together. Here is a YouTube video that shows how to replace the battery.
If the car won't start due to a low battery, press the nose of the key fob against the start button (the end opposite the key ring with the Nissan logo).