I am submitting this as a caveat of sorts to those who may come through this forum looking for tips and advice before tackling a mechanical repair on any G37 convertible.
I have extensive experience diagnosing and repairing just about every problem with this system. I have seen what happens when you try to put the top up in a very low garage, and what happens when you forget your gym bag in the trunk. This is a very simple, but very complicated and very expensive system to repair so accuracy and patience are paramount.
There are 23 steps in top opening sequence, a lag in response or signal loss in one of ~50 inputs the computer is watching for during top operation will cause the system to stop. If a sticking valve or sensor is keeping the computer from proceeding from one step to the next, the system might resume function (temporarily) after the ignition is cycled. Diagnosing almost any fault seems near impossible without access to a Consult III+ and a solid grasp of the system. There are a few good techs I have worked with over the years who lose a days pay with these issues.
Relatively minor repairs like gasket or seal replacement, headliner and cable replacement, or replacement of the infamous rubber grommets should be able to be done without computer support, however that may not end up being the case and the top may become stuck where it stopped. If something goes wrong and you need to have it towed to a dealer. I would strongly suggest using the hydraulic release screw and slowly closing the roof and trunk and having the car set on the roll back with the car's nose towards the rear of the truck.
There are two things I recommend to all convertible owners I meet. Get a very good extended warranty, and keep the battery charged.