Like any animal, lizards at times in captivity will need to be force fed. There are situations and conditions which will result in a lizard not eating on their own.
Some diseases lizards can get will result in seizures, or a similar condition. It is imperative not to feed them during an incident like this. They will most likely not be able to swallow the food. It is best to seek the help of a veterinarian at this point. The animal will need to have a tube placed into their stomach to feed. If the animal shows no signs of seizures, they can be fed using a syringe. It is tricky to get their mouth open, but should be possible using the syringe itself.
Using the syringe, squeeze the processed food into the back of the throat, taking great care to avoid the trachea at the back of the tongue. Make sure to only offer as much food as the animal can handle. Larger lizards can obviously handle more food. It may also be possible to force-feed feeder insects. If the lizard is having digestive problems however, the liquid substitute is much easier to digest, as much of the work is already done. Another advantage of liquid feeding is the fact that you control exactly what the lizard is getting into its diet.
For the vegetable part of the liquid diet, vegetable baby food works very well. This food is high in vitamins and can easily be digested by the lizard. Make sure to add in a calcium supplement to the food. Younger lizards need to be fed more often but in smaller quantities, while larger lizards will need to be fed less often, but with higher quantities per feeding. You have to follow the natural way your lizard eats its food, as not to upset their natural metabolism. If your lizard eats both insects and vegetables, you can use a type of cat food (used for sick cats) if they will eat it. You can also try to add insect matter (use fresh insects) into the liquid mix.