It's possible that there's something wrong with the water or the tank. You can buy pH papers and test the pH levels of the water to make sure it's not to basic or acidic. Clean out the tank and put in a filter. Also, make sure the temperature of the water isn't too cold or warm, since the state of it can be affecting your fish's health and activity.
You should also check the fish food, it might be out-dated, or just not something that beta fish eat. Finally, it's possible that the fish is new to the environment and needs time adjusting. You might be hovering around the tank too closely, which may spook the fish and make it hide instead of eat.