There are a number of reasons why your tortoise may not be eating a normal amount of food or have a lower energy level. Some of them are transient and may be normal while others are more concerning and warrant a veterinary evaluation. However severe lethargy is concerning and is more likely due to an underlying medical, environmental or metabolic problem especially if environmental conditions are normal.
Tortoises may not eat normally due to emotional or physiological stress. Changes in environment such a new tank or changes in décor, or changes in lighting, temperature or diet might trigger temporary disinterest in food.
Tortoises kept with others may be bullied by a more dominant individual. The stress or physical intimidation may prevent a more submissive tortoise from eating. Close observation of interaction between animals is important and this may require separate feeding bowls, tank partitions or even separate housing.
Improper lighting (too little or too much visible light or UVB light) or temperature (too low) or humidity (too low causing dehydration) can negatively impact appetite and energy level.
As cold-blooded animals, tortoises require sufficient temperatures for proper digestion. Most tortoises not only require a gradient of temperatures in their cool to warm but also a hotter basking spot in their enclosure. (Providing your type of tortoise would be helpful to assess and make recommendations about environmental temperature and humidity.)
Before and after shedding, appetite may be down. Shedding is energy intensive and potentially itchy and uncomfortable so while they will commonly not eat during the shedding process, their appetites may be depressed just before and after shedding as well.
Some abnormal causes for appetite depression and lethargy include improper environment (such as temperature too low), mouth rot (infectious stomatitis), trauma, infection (viral, bacterial, parasitic), nutritional disorder (chronic malnutrition, improper diet metabolic bone disease, metabolic bone disease, Vitamin A deficiency (hypovitaminosis A)), urogenital disease (bladder stones), reproductive disorders (dystocia or egg-binding), respiratory infection, indigestion or maldigestion, and gastrointestinal impaction.
The bot***** *****ne is that if your tortoise is otherwise acting and looking normally and the appetite depression or low energy is temporary then it may be normal. However, if the tortoise is displaying any other signs of illness such as severe lethargy or weakness, sunken eyes, skin discoloration, abnormal defecation or urination or the depressed appetite and energy level persists more than a few days, a visit to a local veterinarian with experience in reptile medicine is warranted.
To help find a local veterinarian with reptile experience, here is a useful website you can use to search for a local reptile veterinarian. These veterinarians are active members of the Association of Reptile and Amphibian Veterinarians, which means they have interest and experience in treating reptile patients:
(please note this site may not work on older browsers like Safari)
Please note that since you are outside the US, you will need to choose Advanced Search.
Advanced Search: Step 1 of 2, Member status
Select/toggle honorary member, life member, new grad, active veterinarian, and associate.
Advanced Search: Step 2 of 2
Use the drop down menu in Country to select your country. Press Search.