Dust out the lock's case and spray it with a penetrating lubricant, such as WD-40, as shown. Replace the cover and make sure the key throws the bolt from both sides. Reinstall the lock in the mortise, screw on the faceplate, and replace the spindle and knob. Then put your key in a nice safe place.
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Bench grinder (optional)
1. Penetrating lubricant, such as WD-40