Summary
It is possible for a keypad key cover to come loose over time due to:
Regular wear and tear
Inappropriate handling, including using keys to pry the keypad away from Sidephone as opposed to the intended lift
Magnets that have been installed into the keypads with the incorrect polarity
Steps
To repair a keypad key cap that has fallen off:
Purchase glue from your local department or hardware store that is designed to bond with plastic / silicone / rubber
Place a small portion onto the keypad cap and underlying key
Press the two together firmly until the keycap is firmly affixed, and even with the surrounding keys
Leave to cool and dry overnight
When to Contact Support
Contact support if the keypad came loose from the factory.