Perhaps the most common reason for any keys on keyboards to stop working are due to physical damage or debris hindering the key press. Other common reasons include old drivers, software issues or firmware issues.
Cleaning a keyboard is fairly simple. Start by shutting down the computer or, if the keyboard is wired, unplug the keyboard. Next, clear out excess debris between the keys with compressed air. If the problem persists, gently remove the problematic keys with a flat-head screwdriver or butter knife. Clean the key and areas around it with a damp cotton swab. Typically, rubbing alcohol cleans better than water. Since not all keyboards are designed for button removal, be sure to consult the device manufacturer before removing keys.