Your locking doorknob can become loose easily thanks to the everyday motion of turning it. This is a problem common to every home with a frequently used twist-style keyed doorknob, but you can fix it in a matter of minutes, even if you have no prior experience in using basic tools.
Lining up the Doorknobs
The two hand knobs in most pairs of doorknobs are connected to one another with a long screw or two through the door. In the middle of the door you'll have your locking or latching mechanism. In most cases, a door lock becomes loose simply due to the over-and-over torquing of turning the knobs. Over time, the screws work loose. You will need to line up the two halves of the doorknob assembly to make it easier to tighten the screws. If you're working by yourself, tape the knobs up to the right position using painter's tape.
Tightening the Screws
Using the appropriate type of screwdriver, tighten the screws in a clockwise motion. If the screw doesn't feel like it's catching in the other side of the knob, you may need to adjust both sides until it glides into place. Another issue may be that the screw is broken. In this case, unscrew it completely, examine it and, if necessary, replace the screws with new ones. You can take the old screws with you to the hardware store to find the right replacements. Note: Both screws might not be the same length.
Preventing the Problem
Avoid over-torquing doorknobs when you open doors, especially if they are older or antiques. When you notice a slight looseness, fix it immediately rather than putting it off. If you put it off, you risk issues developing within the lock mechanism. If you have a home with small children, it's all too common to have them hanging off of doorknobs. This may mean frequent tightening. On the other hand, over-tightening screws can strip the threads. Tighten the screws only until they are flush with the doorknob.
Lock Mechanism Problems
If the doorknob is still loose, there may be an issue with the lock mechanism, particularly if it has been loose for a long time. It may have slipped. Remove the screws and both knobs and inspect the interior of the lock mechanism. If anything appears to be broken, you will need to replace the entire doorknob assembly. If it's just loose inside, guide the pieces into the proper position (they'll fit perfectly where they are supposed to be), then reassemble the knob.