I just lost all electric on my car, its dead absolutely nothing.

I have just driven home with no problems, I parked up and got out my car and went to bring my bins in (bin day). After bringing the bin in I press the remote key fob to lock the car but I notice no response. I open the car door and notice no interior light so I get back in the car insert key and press start and there is nothing. Absolutely no electric, car radio not working nothing.

Where do I start to find out what is wrong ?

Surely a dead battery would not be like that.