First, stop forcing it
A key that will not turn is a key under strain. Twisting harder is how a stiff lock becomes a key broken off inside it. Take it out and check the causes below.
Is it the right key?
It sounds obvious, but copied keys on the same ring look alike, and a recent rekey leaves old keys that still slide in. Check the key against one that you know works, if you have one.
Is the key worn or badly cut?
Compare the key with a spare. If its cuts are rounded, or it is a copy of a copy, it lifts the pins nearly but not quite to the right height, so it goes in and will not turn. A fresh key cut from the code or the lock fixes this; another copy of the worn key does not.
Is it the door, not the lock?
If the key turns when you push or pull the door, or when you lift the handle, the lock is fine. The bolt is binding on the strike because the door has dropped or the frame has moved, and the key is being asked to drag the bolt. Houses on hillsides and wood doors in the damp months do this most. Adjusting the strike or the hinges fixes it, often in minutes; our lock repair page explains how.
Is the cylinder dirty or dry?
Outdoor locks, gate locks especially, collect dust and grit, and heat bakes old lubricant into a paste. A dry lubricant made for locks, sprayed into the keyway and worked in with the key a few times, often frees it. Avoid oil and general-purpose sprays, which collect more dust.
Is something broken inside?
If the key turns partway and stops, or turns freely without moving the bolt, a part inside has failed: a broken cam, a stripped tailpiece, a spring. That is a repair or a replacement, not a lubrication job.
A car key that will not turn
In a car ignition, a key that will not turn is often the steering lock holding it. Turn the steering wheel gently one way while turning the key. If that does not do it, the key may be worn or the ignition may need attention.
When to call
If the key is fine, the door moves freely and lubrication did not help, the cylinder needs looking at. If you are locked out because of it, that is a lockout, and it is opened like any other. Text a short video of the key and the door, which often tells us the cause before we arrive.
Questions people ask
Should I use WD-40 on a stiff lock?
No. It is a solvent and it attracts dust, which makes the lock worse over time. Use a dry lubricant made for locks.
The key only turns if I push on the door. Why?
The bolt is dragging on the strike because the door has moved. It is a door adjustment, not a new lock.
My new copy will not turn but the original does. What is wrong?
The copy is slightly off, often because it was copied from a worn key. A key cut from the code or the lock fixes it.
Can I get in if the key will not turn at all?
Yes. It is opened like any lockout, usually without damage, and the lock is then repaired or replaced.