Sleep deprived!

Kylie used to be the best sleeper EVER! She started to sleep through the night when she was about 5 weeks old & slept like that ever since (until now)! Ever since Christmas she has not been sleeping! I have no idea why one day she went "crazy!" She says that "she's not sleepy" or "I wanna play with my toys" & at night we put her down in her bed & she gets right up and stands at the door screaming & crying!!! And when we finally get her back to sleep she wakes up at 3 & 5 a.m. doing the same thing! It is horrible! I have no idea what happened! I just wish she would sleep the way she used to before!