I've read various references that it is not uncommon that modern passenger jets' computer systems need an occasional reboot to fix some erratic state.
Of course, that is done on the ground and passengers sit a while in the dark before the plane is restarted.
Say an A320, or a 767, or the like would encounter a serious computer problem while at travel altitude. Could a complete reboot of all systems (aka power down and restart the entire plane) be performed while in the air?
Edit: Topic 27629 already gives some information on the subject, that rebooting a few subsystems is sometimes performed in flight, but some problems require powering down the whole plane.