The best thing to do to a lawn that was covered with flood waters is to remove any debris left behind once the water recedes. This could be gravel, limbs, stumps, dead fish or animals, garbage or any other debris left behind on the lawn. These items could smother the grass or prevent it from growing back if not removed. After removing any physical debris, rake up any muck or damaged leaf tissue to increase oxygen availability to the grass and roots. Hosing off any muck or debris may also be needed but make sure not to use too much pressure as this could damage roots and crowns of the grass plant. Once the area is initially dried out and raked, watering may be needed to prevent it from additional stress if rains are not sufficient. About 1-2 inches per week is ideal for grass.
The grass plant will generate new leaf tissue in the weeks after the flood water has receded if it was able to survive. After a few weeks you should begin to see this new leaf tissue emerge. This is a good indication that the plant was able to survive and will continue to grow. During this time the best thing to do is nothing. Try to stay off of the damaged areas as they come back for a few weeks with the exception of watering if needed. During this recovery time other seeds and weeds will probably germinate as well. Be patient with this process as it will take some time. After a few weeks of regrowth you should be able to tell if the grass is coming back or not. Early fall is the best time to seed any areas that do not come back and to weed spray any unwanted weeds that may grow as well.