how can i reset my alarm?

hi,
i have a veritas r8 house alarm, since i had a power failer it seems to have knocked out the way it operates.the alarm keeps activating every time i switch the mains back on and will not reset i have tried repeatedly to set my alarm but it will not accept my normal code. have tried also the engineer code 1234 but it will not accept this either. to silence the alarm i have to switch off at the mains and remove the fuse from the mains electric.