Each aircraft has its own specific set of fail-safe systems. The Fokker 70/100 for example did not need electrical power to fly. From memory it was also pretty easy. As you were physically connected to the engines, electrical power was not required to control them. A backup display was powered by small dedicated generator each engine. All you really lost were a few displays and tea on demand.
More recently designed FBW aircraft, as stated above, have their own redundancy modes built in. The Embraer 170/195 series has three generators (engines plus APU), two batteries that will last for just 10 minutes
and a RAT. In addition to that there is a dedicated battery to power (some of) the aircraft's flight controls for 15 minutes.
The way it works is as follows: Given a sudden dual generator (or engine) failure the RAT will automatically deploy. This coupled with the batteries will mean (allegedly) that the aircraft will remain totally flyable throughout the failure. The batteries will then be used to start the APU once its starting envelope has been entered. Throughout the emergency various system will be automatically shed and even when the APU comes on line not every system will be reinstated. But you will have more a flyable aircraft. This aircraft's engines, like the ones described above, have FADEC and each is powered by its own dedicated generator. Once you have everything under control you land as soon as practicable and change your underpants.
PM