A picture frame may look crooked because its hook is off center, the wire has shifted, or the wall fastener is loose. Correcting the cause keeps the frame from returning to the same angle.
Check the hanging hardware
Lift the frame down and inspect the wire, D-rings, hook, and wall anchor. Replace bent or loose hardware before rehanging.
Mark the center
Measure the frame and mark the intended centerline on the wall with a light pencil mark. Use a level across the top edge rather than judging by nearby trim.
Secure the fastener
Install a fastener suitable for the wall type and frame weight. If the existing hole is loose, move slightly or repair it rather than wedging in a larger screw.
Fine-tune carefully
Hang the frame, check level, and adjust the wire or corner pads. Avoid repeatedly twisting the frame against a weak hook.