unknown, I'd start with some basic trouble shooting. Remove the internal battery for a minute or two. If there are not results with it try to

- check for a stuck key

- reloading the software

- open up the unit and shake it out in case something is blocking the key contacts

- clean the circuit board contacts under the keys with an eraser

After that you are in for deeper testing. you would need a multimeter to check the resistance between the opposite halves of the keypads. For that you would of course remove the membrane. Low resistance between the two halves would indicate that it is shorted somewhere and would be interpreted as a stuck key. Use this guide to get to the keypad.