No power to house alarm

Certainly possible, have you checked the fuse board to see if there is anything labelled alarm? Is there any cables coming out of the box? Could be wired to a spur or something. If you can figure out the power supply then it should normally be ok to keep using although you might need an engineers code to access.

Please call a professional alarm installer. If you just power it up without knowing any of the codes then it my start kicking off and you may not be able.to silence it.

I am a experienced intruder alarm engineer. I can sort this ASAP

The alarm may still be connected to mains power and isolated with the fuse and battery removed. Reconnecting this may bring the system back on, however if it was disconnected due to fault then this might activate the external / internal siren etc. Always best to have an alarm engineer visit to check over the system and assist in restoring it if possible.

Certainly, my recommendation would be to identify the make and model of your intruder alarm system and reset the entire system. It's advisable to restart the system because it allows for personal customization of the settings and removes any stored codes, providing exclusive access to the system codes.