could you explain why?
Economies don't scale instantly - they are built around predictable demand. If you remove both supply and demand sources, it changes the global dynamic drastically and in very hard to predict ways. Everyone that had invested money in the United States (and that's a lot of people - even with the AA credit rating T-Bills are considered one of the safest investments in the world) suddenly lost all of the money they had invested. That is a multi-trillion dollar crash. As we learned back in '08, it doesn't take much to destabilize the banking system. '08 was a huge hit - you've just removed more than 20 trillion USD from the global market capitalization (in 07 the market cap of the rest of the G8+China-Canada is on the same order of magnitude). The global banking system fails. This means that no one is able to get the capital to re-tool for the post-US economy.
Functionally, the systems needed to cope with the change in the situation (the global banking system) are the very systems that have just imploded in the most spectacular fashion possible. This makes the 1930s look like a picnic. This is because the world today is more, not less reliant on banks (and the banks are more interconnected) than they were in the 1930s, and you've just kicked the base out of the system.
As one side note, because we have SSBNs on patrol constantly,
President Spellings* would still have command of a nuclear force. I'd think they would end up at Faslane.
*Selected under the assumption that one of the cabinet secretaries is out of the country.
Edit: Just to be clear - the US doesn't have a single power grid - it has three (Eastern, Western, & Texan). There are fail-safes in place to prevent the entire grid from going down at once.
And if you say anything like HEMP, I will be most disappointed.