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immobiliser not recognising key?

i took keys out of ignition, left headlights on. when i tried to start it 5 minutes later the immobiliser didnt recognise the key and wouldnt start. i have disconnected battery and tried again but no luck. any ideas on how to overcome this problem?
clive ingram, December 2013
Give us a clue - what vehicle?

Concerned parent, December 2013