What do I do on days when I try to play the piano but find my fingers have metamorphosed into spagetti?

This happens to me too. When your fingers go floppy, stretch and mage them for 5 minutes. Meanwhile, listen to your favorite clical music and enjoy! Start practicing your scales after that. Your fingers will recover in no time! Well it works for me! =) Let the music flows in you! And by the way, good luck for your exam!