I was doing some shooting with my 5D, testing the Zeiss 50/2 Makro Planar lens. I was doing some long exposures, the frequency of which is nothing I haven't done before. I finished shooting and turned off the camera to change the lens to the Zeiss 50/1.4. When I tried to turn the 5D back on, it wouldn't turn on. The battery that I was using had only one power cell left, but I did have another with me that I checked before I went out and it was fully charged. I changed batteries, but it still wouldn't turn on. One of my friends suggested that the sensor overheated from the long exposures and to let it cool down for a few hours. That was three hours ago and it still won't turn on. Any ideas?