Car dead in garage

[pupad7]

So drove the gti last Tuesday - all well
Parked it in garage and because I thought I wanted to vent the inside I left the drivers window open slightly
As far as I can remember all doors shut, no lights on, no radio etc
Went to car today and immobiliser light not on and engine doesn’t turn at all

Is it just a dead battery? Any thoughts?

[dragonfly]

How old is the battery? Jump start the battery and check the voltage at the alternator to see if it's charging.