Power plant != power grid.
If you mean only all the power generation plants where destroyed, than it take really little time to rebuild the grid since, all the national grids are already connected (Italy for example already has connections to all the neighboring nations power grids). In this case the power grid will be up and running again in some days, but the nation will be 100% dependent on imported energy for many years
If you mean that only the power grid is destroyed, than it takes months, if not years, to rebuild it, depending on the level of destruction: if only the large transport lines are destroyed, then, in a couple of years, it is possible to rebuild them (but not assured you will do it), if you mean 100% of the grid, which is all of the above, plus the small line that arrive to your home, than you are out of luck, it'd require many years.
If you mean that all the power plants and distribution grid is destroyed, you need years (at least a couple of decades) to return to the previous level, assuming you will be able to survive such an event without other problems: no energy -> no food conservation AND production -> you need to import also the food, for example. Think like a "Berlin Blockade" in the 1948 extended to a nation to have a hint of what it means.