help-immobiliser on and car wont start

[oliveoil]

:zx11::aargh4:I have a golf s 1.4 mk4, w reg. when we bought the car we were supplied 3 keys the main one did not work as battery was dead so we manually opened drivers door and have not bothered to replace it. this morning was the second time in 6 months that the car stopped dead after initial start up. what do we do and what about battery it has been missing out of fob for 3 months.
help!!!:1zhelp::1zhelp: