Depends on the size & altitude of the nuclear explosion. It also depends on the device, some will come back on , some will not.
The biggest threat is to transformers. All long-distance powerlines work as giant antennas that readily take the EMP "Signal" which overloads the transformers. Transformers are easy to make, but not so easy that you can replace millions quickly. During a bad power storm, it's not too hard to destroy a few dozen, each taking a hour or two to replace. A nationwide EMP strike (Or solar flare) would result in the replacement of tens of millions of them. How do you make a transformer when you can't get production started at a factory?
There has been quite a bit of study on the subject, and if North America was to have a solar flare, or a very localized EMP attack that allowed Europe & Asia to survive without issue, it would take 3 to 5 years to restore the grid. If Europe & Asia did not survive, 10 to 15 years to restore the grid.
The death tolls from a North American EMP strike are estimated at 100 to 175 million deaths due to starvation and lack of access to clean water.
