Switch off, remove battery, replace battery, switch on, try again, anything then?
Sounds stupid but it happened to me with a 450D one time. Did the above and all was well and continued to be well for the further 9 months I owned the camera.
My 50D does something similar. I have found taking the batteries out of the grip and then placing them back seems to do the trick. Its only happened twice, once in extreme cold and once in the wet!
You're a brave man
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