If you somehow manage to get on top of the elevator cab (indeed as the sibling comment points out, there is no escape hatch at the top), you will generally see a small control panel with a switch to put it in inspection mode. That turns off autonomous control and from there the cab will stay where it is unless you manually action the up/down switches (assuming the "safety chain" isn't broken - which if the elevator suddenly stops, it may be - so even those won't do anything then).