Back in the days when you could go into the cockpit for a chat, the pilots turned the engines of our 747 off (well, idle) over the middle of the Atlantic to show my brother and I how the autopilot worked.
If you're APU isn't running and you hit the disconnect on all four engines you have no electrical or hydraulic power and there is no way to fix it in the air!
https://www.angelfire.com/ct3/ctenning/electrical_essays/747...
> The pilots control module for the electrical power generation system is in the pilots overhead panel (P5, Figure 3). There are four drive disconnect switches at the bottom of the module. These are guarded switches. Lifting the guard and pressing the switch will cause the IDG shaft to mechanically disconnect from the engine gearbox. This would only be done in the event of an overheated IDG or low IDG oil pressure. Once disconnected, the IDG cannot be reconnected in flight.