I had this problem with my son, he just lost all interest in going to nursery even though he use to love it, I asked them if there was anything wrong and there wasn't and my mum who is a teacher suggested he could be bored and need different activities, so i suggested to the nursery that he be moved upto the next group ( which he was going to go to in 4 weeks anyway) and the difference was unbelievable...he could just be bored.Just walk away during the night he will soon learn that you are not going to give in..
Good luck.