My camera is a Canon 400D (digital rebel XTi). It sat on a shelf for quite some time.
When I insert a (newly charged) battery, the camera doesn't switch on. I get this screen:
and switching the camera on/off doesn't do anything. Without a lens mounted, I get this screen, and I can't turn off the camera; I have to remove the battery to turn the camera off.
Before this problem appeared I replaced the clock battery with a new one, but I'm not sure that it caused this problem.