I have tried to do a firmware update on my 5D Mk2 but the update stopped at 98% until the battery dies. Can anyone help please?
You can try removing the batteries - main battery and the small date/time battery (which should be accessible via the empty main battery chamber) then leave the battery off for a while - I would leave them out overnight so the circuitry capacitance can be completely discharged. Replace the batteries. There is a good chance the camera might come back to life. If not, send in for service.
If the camera comes back, I'd re-download the firmware, reformat the CF card and try again. I would not use the same download which might be corrupted.