From my experience, it seems like every parent I know (including myself) has this exact same problem sometime around their child's first birthday. Their kid goes from being a stellar sleeper to suddenly waking up in the middle of the night. Sometimes their child is upset; sometimes they just want to wake up and play. Sometimes approaching new milestones can cause this; sometimes a growth spurt. Lots of things are going on in your little girl's body and any one of those things could be waking her up, and she doesn't really know how to get herself back to sleep. And she's probably disoriented. I agree with Jaysker. Check to see if she's comfortable. Your body temperature goes down when you sleep, so while she might be perfectly comfortable in her summer jammies at bedtime, after she's slept for a few hours she might wake up cold--especially since the weather is starting to get cooler (and, if she's anything like either of my children, she kicks the blanket off of her). The solution might be as simple as putting on a pair of socks for the time being. If it's a growth spurt, she might be waking up hungry or thirsty.