Because of the sheer magnitude of such a doomsday event, you won't be able to count on agencies such FEMA or the Red Cross for long. Maybe in the weeks just after such an event water and food "could" be shipped in, but relief organizations and government agencies are just not equipped to handle supplying the needs of so many people. You really need to relocate to a long-term bug-out location - and stay there.
In the weeks following a total power grid loss you'll see the best of man. People will probably help each other with food, water, emergency care, etc.
But this disaster event isn't like some that may be resolved after a months - the effects of this doomsday scenerio will last for years. Eventually people will get thirsty, or hungry, or diseased from drinking contaminated water. Then you'll see the worst of man as they fight for survival. You will need to stay safely hidden, and away from people until things have a chance to stablize - possibly years.
I know saying this is harsh, but eventually the population will drop to a point it that can be supported by the available resources - your mission is stay away from others until that time.