What is a collapsed gallbladder and what are its causes?

Don't know. It fills and empties as part of its work. If diseased and scarred to a nubbin that could be called collapsed.
Healthy! The gallbladder's function is to empty it's contents into the intestinal tract in response to eating. A healthy gallbladder will empty almost-completely and may appear contracted or collapsed on ultrasound. It is harder to see gallstones on ultrasound when the gallbladder is contracted. Therefore, it is best to have an ultrasound after a period of fasting when looking for gallstones.