does us government have action plan,what do after an asteroid has already collided with earth?

Hide to save themselves

There are plans for natural and manmade disasters including hurricane, earthquake, terrorist attack, war and other possibilities including highly unlikely ones such as asteroid impact.

Actually, they're working on it:
https://www.sciencedaily.com/releases/2019/04/190424210808.htm

Of course they do not have a plan for when New York is wiped out by an asteroid. What's YOUR plan?

the important plan is to save Nancy Pelosi and Elizabeth Warren - the troubadours of our generation.

You have a complete misconception of government. What would you expect of them, solve everybody's problem? WRONG--
Collectively,,we humans would just have to pick up the pieces and salvage what were possible. First responders, red cross, yea perhaps even the guard and military.
Volunteers from the world over rolling up sleeves to pitch in an help through the aftermath.
Look how we deal with catastrophic severe weather like cat5 hurricane wiping out several states. How bout that Russian meteorite that did plenty of damage.
you think some suit on capitol hill will be getting his hands dirty?? I don't think so.
A 1 Km asteroid woild be like the Arizona Barringer, likely wipe out a whole county, devastate much of the state and DC not far enough away it would go unscathed.
Plan all you want, you won't stop it, just control the chaos in the aftermath.
Read about 9/11, a good perspective.

forget that...imagine europe was going to be wiped out but america would not be...so everyone has to move...how could that work?

They would say don't worry folks its not gonna do that much damage, if it even hits at all....then they would immediately run like Hell to their underground shelter in the mountains.

The impact of an asteroid 1 km wide anywhere on Earth would likely kill all humans.