Found this on another forum. 6D specific:
1. Make sure the camera is off, and then remove the battery, memory card, and lens.
2. Close the battery/memory card doors.
3. Turn the power switch to the "ON" position, and then press and hold down the shutter button for 10 seconds.
4. Release the shutter button, turn the power switch to the "OFF" position, reinsert the battery, memory card, and attach the lens.
5. Power the camera on.
Failing this or finding any corrosion/moisture, etc. in the battery compartment it looks like you might need to send it to Canon for an evaluation.
Sometimes not taking a photograph can be as problematic as taking one. - Alex Webb