Often something has spilled into the keyboard.
Pull the battery & disconnect the power. Then use some distilled water to wash down the keys use some can'ed Air to blow off the excess water. Let the system fully dry before powering (a good 24hrs). If that didn't unstick the key you'll need to open the system and go deeper in the cleaning. Don't use tap water as the minerals in the water can cause corrosion or deposits that harm your system. Isopropyl alcohol can also be used.