Showing Error 5 all the time. Can't take pictures

This will take longer to write then to do it. Error 5 in my experience can be memory card, internal flash or actually an unlocked lens. The lens unlock button is pretty big and sometimes gets pushed in as my clients are zooming or twisting the barrel somehow. What I've had to do is shut the camera off, remove the battery. Unlock the lens ( I wait a second or two) then lock it back in place. Another problem was having an external flash installed and hit the internal flash button. The number 1 problem I've found was the Memory card, either it was (is) a new card that hasn't been formatted to the camera, a memory card that was formatted in the computer (that really messes with their brain) and another thing was continually using the delete button on the camera, or if it's a new card and you have formatted it and formatted it again and it still doesn't work try another card.
Okay the last suggestion and what actually happened to me on my Canon EOS 1D Mark II and III I was using 4gig cards bought 8 gig cards and went nuts cause they wouldn't work. Went on Canon's site and downloaded the updates which ended the problem.