As the weather changes, it is not unusual for doors to stick or to not close. Wooden doors swell as humidity increases. The door frame changes shape and size as a result of humidity and ground movement. However, something as simple as a loose screw may be the culprit. If your door will close, but it won't latch, see our article on doors that won't latch.
A wooden door that that sticks or won't close is usually fairly easy to fix. Other types of doors can also be fixed, but there are fewer options when it comes to modifying the size or shape of the door itself.
The first and easiest thing to check is that all the hinge screws are tight. If the door is sagging, prop it up first with a magazine or book before tightening the screws. If the wood won't hold the screws tightly, you can drill it out and insert a piece of dowel or for a quick fix, fill it with toothpicks and break them off flush with the surface. Check that the door latch and door knobs are secure too.
If the door still sticks, there are a couple of things you can try. It may be possible to enlarge the door opening just by driving a couple nails through the door frame and into the studs underneath. This has the affect of snugging the door frame up to the studs. It may only move 1/16 of an inch, but that may be enough. Find where the door sticks and drive a 2 to 3 inch nail into the door jamb at that location. You might need another nail 6 to 12 inches above or below that spot. It might be possible to do this on the opposite side of the door jamb but it is likely to be less effective because the weight of the door pulls on that side.