I had this problem after messing with the camera whilst not powered down. The conclusion I came to after a few hours of research and tinkering is that I created some sort of short in the camera part of the board and never managed it to get going again.
I tried changing the ribbon, the camera, the card, the power etc but I just made a mistake and killed the pi. Needless to say I always power down now.
There is a multitude of info out there for trying different things give them a good go first, you never know.