How can I fix my house intruder alarm?

8 years ago a Compact SL8 intruder alarm (very basic key pad) was fitted. Internal battery Texecom 12V 2.1 amp.
Last night a general power cut in the area put the alarm completely out of function as battery was old, hence no power was in storage.
Hours later when the power was restored the alarm went off, tried to stop the noise according to set code however, without success.
The only light on the key pad was the red one indicative to power. The only way to stop the noise was by switching off power to main box completely.
Today I replaced the battery however, I am still in the same situation: alarm goes off as soon as battery is connected or main power provided or both. Only light on key pad still red for power but not geen day light and the only way to get rid off the noise is by disconnecting the battery and/or switching off the main power to the box.
Please could someone help me with this problem to restore normal working system?