Once the batteries are out, toggle the power switch on & off about 10 times.Go ahead and replace the clock battery (and honestly, this is probably a good time to replace it if you haven't), and then insert a fully charged camera battery.
For some reason (I'm told) toggling the power switch on & off a bunch of times in a row helps clear capacitors in the camera and flushing on board memory. If this method won't works for you then the only thing that can work is hard reset. For hard reset details and other fixes you can use Canon 5d Manual for better results.
