R33 Won't start and electrics die out

[Bettiboop]

Howdy

I have got an R33 RB25det ( GTST)

 

i have had some bad luck and have parked the car away for 12 months and unfortunately, on the first-night rats have got to the interior electrical so to be intact apart from head unit wiring which isn't big issue due to i just want it to start etc

I go to start it and I believe the starter motor is clicking the solenoid but isn't turning the engine on at all and then the dash lights turn off / every electrical including head unit etc

 

then when i take the key out and retry the same thing happens 

Unsure where to look at any suggestions any common faults I should be looking into?

 

Thank you in advnace

[GTSBoy]

This would be a flat battery.