Logic 4 will not set with the key?

Hello. (This has been a problem for 5 yrs now, so we haven't used the alarm system )When I try to set the alarm by turning the key the buzzer sounds but the system doesn't 'arm' and the buzzer never stops . I have changed the back-up battery ( 5yrs ago ) thinking that may be the reason for the problem, but it wasn't . Thank you.