The key is usually jammed by debris. Make sure the laptop is turned off. Turn it over and clean it with compressed air. You can buy compressed air tanks at most office supply stores. Make sure to place the box vertically, otherwise you will spray compressed liquid, which will freeze and damage the keyboard. If the compressed air tank is insufficient, try to use a higher pressure air compressor.
They produce non-conductive lubricants. Sometimes, spraying a little water on the keyboard will release a stuck key, but it should always be used in combination with compressed air. In other words, do this only if compressed air is not working. After applying lubricant, turn the keyboard over again and remove excess lubricant.
Most keyboard keys are movable. You can carefully release them to clean the area underneath.
If it fails, you can replace the keyboard. Usually, you can find them at reasonable prices on eBay.