Lol @ cat causing the car not to start, that's a pretty far out idea lol don't think that'll ever happen...
Bro answer our question properly, is the engine cranking over? You said it is then you say it's like you've got a flat battery? Huh?
CASs do go from time to time, old RB30s suffer from that badly, infact it's not a rarity in any RB...
Try uploading a video of it trying to start, cos from what you've described, it could be almost anything. And some info on the car would be nice lol, we can't tell you anything unless you give us details of the car. What mods, auto/manual, kms, aftermarket ecu or stock etc...