I have a Canon 5D Mark III which recently would no longer wake up from Auto Power off.
Canon Support suggested clearing all camera settings and all custom functions.
That did not resolve the issue.
I removed the main battery and left it out for 30 minutes (i.e. got distracted). After putting it back, the camera will now start up again after Auto Power Off with the 1/2 press of the shutter release button
which is good because the camera is now 3 days out of warranty.
My next step would have been to remove the date/time button battery as well.
Hope this helps someone.
