Depends on the size, of course. It would take more than 1 to cripple the entire country I would assume, but yes, if enough went off the country would grind to a halt. Cell phones, ATMs, etc would not work, at least for a time.

There are tons of books about this. http://www.amazon.com/One-Second-After-William-Forstchen/dp/... was a good one. Fiction, but still.