Won't power up with new batteries - Canon PowerShot S2 IS Digital Camera

Assuming that the new batteries are the correct type for the camera and that they are fresh/fully charged, your problem may be due to corrosion on the battery contacts inside the camera which can prevent the full power of the batteries from flowing into the camera. Try this free fix before you do anything else: remove the batteries and wipe the camera contacts firmly with a dry cloth (heavy corrosion may require cleaning with a wire brush, steel wool, or sandpaper). Remove any residue that may have fallen into the battery compartment during cleaning, then wipe both ends of the batteries and place them back in the camera. This cleaning clears the problem about 90% of the time. If it doesn't work for you, your camera may have a problem that requires professional repair.