If a door in your home unexpectedly fell off its hinges, it's most likely due to wood damage on the door. Wood damage can be caused by a variety of factors, including slamming and banging or the wood rotting due to water exposure. When a door falls off its hinges, you can generally return it to the same hinges, but first you must repair the damaged area on the door, or the door will just fall off again once you reattach it.
Remove the door from the hinges completely if it is still attached in any way. To remove the door from the hinge to which it is still attached, unscrew the screw or screws still holding the hinge to the door, then place the door on a flat surface.
Repair the wood in the area where the door came free of the hinges. If the door has only a small amount of damage where the hinge came unattached, apply wood filler to the area with a putty knife and let the wood filler dry completely. If the door has a large area of damage, use a saw to cut out the part of the door that is damaged, mix up a batch of wood epoxy, and fill in the entire area with epoxy. At the surface level of the door, form the epoxy with your fingers so that it is in the original shape of the wood that you removed. Allow the epoxy to dry completely. Fill in the screw holes from the hinges while filling in the damaged area.
Sand the damaged area where you used wood filler or epoxy with a sanding block until the wood filler or epoxy lies even with the surrounding wood. If the filled area is in a location where it will be visible when you hang the door, paint the filled area with a colour that matches the door.
Check the doorway for structural damage. If the hinges stayed in place in the doorway when the door fell off, the doorway's structure is most likely sound. If the doorway cracked or tore apart when the door fell, you must also fix the wood of the doorway before you put the door back up. Use the same method you used to fix the wood on the door to repair the doorway.
Remove the hinges from the door frame, whether the frame was damaged or not, and move the hinges upward or downward by a couple of inches. This ensures that the hinges will be attached to an undamaged spot on the door.
Return the door to the door frame and reattach the hinges. It helps to have two people working together when attaching the door to the hinges, with one person to hold the door up while the other person attaches the screws. To reattach the door, simply line the door up in the doorway and attach the hinges where they hit the door.