What is HEMP? I'm laughing as I type this, but do you mean pot, or what?
Haha, no - HEMP is the acronym for "High Altitude Electro Magnetic Pulse".
http://www.todaysengineer.org/2007/Sep/HEMP.asp
In rough summation, anything electronic would be fried. There are exceptions, of course, but a HEMP attack would pretty much move us back to the 19 century instantly. Best estimations suggest a relatively small (1.4 megaton) nuclear blast at an altitude of about 200 miles (that's the tough part) would basically destroy all electronics in North America instantly. That being said, a smaller yield weapon could be detonated as low as 20 miles in altitude and would be capable of crippling up to 40% of the continental US.
Imagine all communications, autos, computers, even electrical service - anything electronic - is suddenly useless. How long before post-Katrina looks like a walk in the park?