Most lizards don't have huge appetites and eat sparingly, which can make it difficult to tell if your lizard is eating like he should -- especially if he's housed in a group. The two best ways to tell whether he's eating are to weigh him regularly and look for feces. A sudden drop in weight or a lack of feces indicate a problem that should be investigated by your veterinarian.
Molting, changes in the light and hormones can all cause your lizard to stop eating or to eat much less than usual. Molting occurs once or twice a year, depending on the species. Your lizard is likely to go into hiding and avoid food for a few days. If your lighting simulates winter conditions, or if he's in an enclosure where he sees a reduction in daylight, your lizard's appetite may decline. During breeding season, some lizards may not be interested in eating, especially males.
If the temperature drops too much, your lizard may stop eating since his body functions will slow down. Monitor the temperature in his enclosure with a reliable thermometer, and make sure the temperature is within the recommended range for his species. A dirty enclosure, a sparse one where he has no place to hide and feel safe, or overcrowded conditions can also cause your lizard to stop eating.
Sudden changes can stress your lizard, making him hesitant to eat. Stressful changes include a move from one enclosure to another, addition of another lizard, rearranging items in his enclosure or moving his tank to another location. Make changes slowly if possible, and give your lizard a few days to adjust to the new situation. If you have more than one lizard in an enclosure, observe them to make sure one isn't being kept from the food.
Illness, Infection and Other Problems
Any sort of illness, from a skin condition to kidney problems, can cause your lizard to stop eating. Parasites and external or internal infections may also diminish his appetite. Some lizards are prone to becoming impacted, and unable to produce feces, especially if they aren't getting enough water, which will in turn cause a loss of appetite. A change in diet is usually necessary to relieve the impaction.