My Canon 5d developed a strange problem.
When I turned it on none of the function or shutter buttons would work for at least 30 seconds. Sometimes more.
I was on the verge of sending it to Canon for repairs when I thought of the small watch battery that keeps the date and time. When I pulled that out, the camera worked fine. Which leads me to believe the battery was shorted. Now that I have replaced the battery, I have my 5D back!!
So, if you have any strange issues with the camera, check the date/time battery first.