Depends, but sounds like either you have a loose hinge that has allowed the door to get out of alignment or possibly the hinge plate either on the frame or the door is loose. If it is just the hinge plate loose, i.e. some screws have worked loose, then you just tighten the screws, You might have to have someone lift on the door from the door knob side a bit to get it back into alignment before you tighten the screws. If the screws have stripped out of the door frame or door, then you will need to repair that. Generally getting some carpenters glue (almost any home supply, hardware store, or even a drug store will carry it.) Take some toothpicks, and dip them into the glue then stick them into the holes where the screws went, and keep sticking them in until the hole is filled up. Break off the part that sticks out. You can use a utility knife to assist that or to trim away excess. Let it dry, then put the screws back in. It will hold tight. If might be that there has been some settling of the house and the frame might have warped a bit. If that you need to probably put a spacer behind the hinge to push it out a bit (most likely on the lower hinge if the little closing hasp (knob looking thing) is too low, or on the upper hinge if it is too high. You can make them out of the kind of cardboard you find on the back of a legal pad or tablet. Just cut one about the size of the hinge plate and put it between the wall and the hinge plate and put the screws back in. If it isn't enough, add another one. If it seems a bit too much, then just tighten the screws a bit, or you could remove it and try to find a bit thinner piece of pasteboard, or carefully shave a layer or two off of it.

Remove the hinges, fill the holes with repair putty, and when dry, re-hang the door.