My gallbladder is collapsed, why did this happen?

This is normal: the GB stores bile that is made by the liver. In response to a fatty meal, the GB squeezes and empties the bile into the intestines to help digest your food. A collapsed GB just means that it has recently emptied, probably after a meal. Conversely, a distended or full GB is common when fasting.

Symptoms?: Are you having any symptoms ? I assume you had pain if they ordered an image of your gallbladder. It depends on how severe , how frequent your attacks are Some manage with diet, weight loss etc. others have surgery to have it removed. If you are not having symptoms you can probably just watch it. May want to repeat image in 6 months to ensure it looks normal.