Setting alarm?

I have a Paragon Plus alarm system.
It is fitted and has been working correctly for afew years.
Resently I am unable too set the alarm. When I attemp to do so, the alarm light indicator on the panel is lit and the panel creates a dull alarm setting tone.
I have walk tested all the sensors and they are all working correctly.
I would appreciate it if someone could suggest what the problem could be. Thankyou...