Cabinet doors sag when a screw or hinge comes loose, when the hardware is damaged, or when the wood underneath the hinge is stripped or gouged by a loose screw. You need a screwdriver. (A helper makes it easier to hold the door in position while you work.) Here’s what to do:
Open the door.
If the top hinge is loose, push the door up and back to straighten the door.
The bottom hinge rarely goes bad because there’s no pressure on it, but if the screws are loose, tighten them.
Tighten the screws while your helper holds the door in that position.
If you don’t have a helper, prop it up with your foot and lean into the door while you tighten the screws.
Let the door hang by itself.
Open and close the door several times to make sure the screws don’t work loose right away. If they do, then the screw holes need repair.
If you can’t get the hinge secured, back the screws all the way out, take off the hinge, and look at the wood beneath it. If the hole is too large or has been stripped, you need to fill it in and then drill a new hole. You need a drill, a screwdriver, toothpicks, and a liquid white glue such as Elmer’s. Here’s what to do:
Squirt some liquid glue into the hole.
Stuff the hole with toothpicks until it won’t take any more, and then cut or break them off flush.
You can repair surface mars or dents with wood filler, but filler won’t hold screws. They need to be inserted in wood, and if the hole is too large, it should be filled with toothpicks first.
Drill a pilot hole through the toothpicks, using a bit that’s much smaller than the circumference of the screw.
With a screwdriver, put the screw back in. If the screw is stripped, put in a replacement that’s the same size.
Let the glue dry.
After your hinges are secure, keep them that way. Periodically tighten the screws, but don’t overdo it. And lubricate the hinge. A drop or two of WD-40 on each is sufficient. You don’t want the lubricant dripping down the doorframe.