I know exactly how you feel. My DD did this too and I was at my wits end wondering whta had happened. Then all of a sudden she starting sleeping through again.

It's a stage some go through, my DD would wake up between 3 and 4 and be playing and singing in her cot in the pitch black. I wouldl just leave her and she would go back to sleep, although some nights it could take an hour or so.