How can I fix Accenta ADE alarm?

Hi, Hope someone can help. The electricity supply in my flat was turned off for a few days. When the supply was turned back on the Accenta ADE alarm started sounding and we could not turn the alarm off with existing code. (The alarm had been working fine previously). We've been through the manual and can't find any solution. We have had to remove fuse to stop the ringing. Thanks.