Restarting a down grid is nontrivial. Small power stations are used to kickstart the larger stations. Think of the starter motor in a car. Now do all of this while precisely maintaining 60 Hz (in the US) everywhere.
Can they still do it the old fashioned way with a telephone to coordinate and an oscilloscope to line up the phase?
Back in 2014-ish, when Eskom in South Africa was having particular problems with blackouts, they discussed this possibility. Some estimates on restart times were given at the time, but this is the only reference I could find now:
https://africacheck.org/reports/does-south-african-face-an-e...