Considering your job, it's likely you have disuse muscle atrophy. Adequate protein intake is essential for building muscle. Eat good quality protein such as eggs, chicken, beef. You might want to try whey protein powder and creatine.
Start going to the gym and lifting weights and try walking as well. Exercise increases serotonin (happiness chemical), dopamine (low dopamine levels cause addictive behaviour) and endophins (feel-good chemical).
Tyrosine (an amino acid used by cells to synthesize proteins) is the building block of dopamine. Some of the foods that contain tyrosine include almonds, avocados, bananas, beef, chicken, eggs, milk, watermelon, yoghurt.
Take a good quality multi vitamin - whole natural product if you can find it. Also take Vitamin D3 - not only aren't you getting enough vitamin D with your job due to lack of sunlight but not enough vitamin D leads to muscle and bone weakness and increases the risk of many conditions and diseases from cold/flu to cancer.
The other type of muscle atrophy is called neurogenic atrophy which is caused by an injury or disease that affects the nerves connected to a muscle. This type of atrophy is mentioned to appear more suddenly and is more serious than disuse atrophy.
There are various reasons for neurogenic atrophy such as Lou Gehrig’s Disease, Multiple Sclerosis, diabetic neuropathy, neck or spinal cord injury to name a few. You might want to see a doctor to rule out any underlying diseases/injuries as well.