Veritas security system?

I turned my power off completely to change some lightfittings. On turning the power back on, the keypad beeps but will not accept the code to disarm. After about 30 seconds the alarm goes off and the only way I can stop it is to pull out the fuse that the system is on. I have looked in the main board and the fuses are intact and there are no loose wires. What have I done and can anyone tell me how to fix it please????