Hi there!
Muscle atrophy is a medical term and should not be confused with loss of muscle tone and bulk experienced after a period of disuse. If there has been an increased gap, exercises would need to be started with gradual increase in duration and load. It may take as long as 3-6 months to appreciate a visible difference and around 6-12 months for the tone to return to normal, depending on the results you are expecting. If you are actually talking about muscle atrophy, exercises are better avoided and one may consider an evaluation be a primary care physician to look for secondary causes and appropriate management.
Hope this helps.
Take care!