Question for any petrol heads on here...

I left my car outside my mums all day today went back to it tonight and the keyfob control doesn't work, usually you press it and the car's indicators flash car opens or closes etc, no problem i can get a battery for it tomorrow i thought, i'll just open it with the key and drive home, but no, the car is completely dead, so my question is, is the car immobilised because i can't unlock it via the fob? or is it just something simple like a dead car battery?

The car is a 2006 Vauxhall Astra estate by the way.

Cheers.