can not reset alarm system / accenta 8 mini optima?

Hi, can not reset the above alarm due to a tamper
i have opened the control box and found that the
battery is over 10 year old.i have replaced a new
battery today but this has not helped it still will not take any codes.the alarm would sound if connected
to battery or mains.the alarm system was okay before the tamper.i think i am locked out of the system. can any one help thanks.