The short answer is gentle. Keys are not very thick and if the cuts on your key are deep, that only makes the key more likely to break. If the key or lock is old, keep in mind that metal on metal wears down over time. So it's not surprising that your key is stuck. In general, a lubricant such as Ballistol or WD-40 is great for locks. A little lubricant can go a long way for the longevity of your mechanical locks. We recommend you try to gently turn the key to the 12'oclock position and try to pull the key out. If it does not come out, please call us. We can extract your key using specialized tools. Then we'll figure out why it got stuck in the first place and repair the key and lock to perfect working order.