Firstly, does the handle lift up and key turn to lock when the door is in the open position? If the answer is yes, then the issue is the door has either dropped or out of alignment. 

If it doesn't lock in the open position, then you have a hardware issue or failure. 

If it has dropped then try carefully lifting the door upwards whilst attempting to lock, ensure the travel of the handle is the same as when the door is in the open position.