There are a few reasons why your power liftgate may not be working. The most common reason is that the battery is dead or dying. If your car's battery is old, it may not have enough power to operate the liftgate. Another possibility is that a fuse has blown. Check your car's fuse box to see if there is a blown fuse for the liftgate. If so, replace it with a new one.
Another possibility is that the power liftgate motor itself is faulty. This can happen if the motor overheats from excessive use. If this is the case, you'll need to replace the motor with a new one.