Hp photosmart 812 - no power

Taking a camera from a near freezing environment into a lovely warm house can cause condensation to form inside the camera. Mant cameras are designed to close themselves down if they sense any moisture so I suggest leaving the camera in a nice warm air flow. Not hot, just warm and leave the batteries and card out and the doors open to allow some warm air to circulate inside the camera.
It could take some time. Also make sure the battery contacts aren't corroded or dirty and make sure the memory card is clean too.
If that doesn't work I'd guess some damage had been caused by the moisture.......

BMW