The hinges are the key to holding a door properly in its door frame. If the hinge comes loose or the screws pull out of the wood, the door starts sagging and may even fall off from one corner. While this is a problem that requires immediate action, it is not an issue that usually needs professional intervention. Fixing a falling door just requires some basic building materials and a little bit of time.
Ask a helper to lift the door so it properly fits into the frame with its hinges lined up to the holes in the door frame.
Inspect the holes in the frame into which the hinges attach. If the holes are damaged, glue them with an epoxy to strengthen the area around the edges. This renews the strength of the wood around the hole so it can once again support a screw and the weight of the door. Wait several hours for the epoxy to dry before continuing.
Drill the hinges back into the holes in the frame using a slightly larger screw than what was previously used on the door. The larger screw helps ensure that the hinge attaches tightly to the frame and the door fits more securely as a result. Another option is to coat several toothpicks with polyvinyl glue and then hammer them gently into the holes to provide a tighter fit against the door screws.
Open and close the door several times to make sure it fits properly in the frame.