It takes power to get any plant running, nuke or fossil. Pumps, motors and fans have to be going to lite a fire, get water flowing to the boiler, etc. Most plants, especially larger one, have on-site diesels for just that purpose. It's called a 'black-start'. In this situation, most of those plants that had to cut load probably rolled back to "hot stand-by" mode --- producing just enough juice to meet the plant needs and they should be able to ramp up power relatively quickly. The trick in a situation like this is bringing them on in the right sequence at the right power levels to keep the grid ballanced and gradually return service area by area. That can take a long time. The dispatch and plant operators are all pretty busy guys right now.