Try to reset the system to default settings, probably by a jumper switch inside the panel. Then enter engineers mode (default) and you can set the passwords etc. You'd really need the instruction manual for jumper settings and default passwords etc. As to why the alarm sounded with power off. It could be that the system has reset itself and is reverting to its default setting of sounding alarm in the event of a power cut (i.e. in case someone trys to cut cables to bypass alarm).