If this is a recent phenomena, then we will assume your hinges are in good shape and that the door is hung correctly. Now, take a look at your striker plate and determine if it has slipped back some so that the door latch won't grab it. Use a big Phillips screw driver and loosen the bolts, then move the plate toward the outside as much as possible and then tighten down the phillips head bolts. If the bolts won't budge, then get a pair of vice grips and clamp onto the handle of the big phillips screwdriver to give you some leverage. That oughta do it. If not, then get back. Your striker plate may be worn out, or the hinges may have worn and have put your door out of alignment.