If you stop working out your muscles will atrophy (you have about 3 weeks before this happens). People will gain fat when they stop working out because they don't reduce the amount they eat enough when they lose muscle. Also, the body will break down the muscle and use it as energy or used to constructed other molecules. Muscle will also naturally decrease with age which is why it is important to do some strength training throughout ones life.
They go down if you let yourself go(obviously by not working out for a week isn't gonna do something).