Fiat Punto immobiliser

Hi - Your advice requested,
I recently had a starting problem with my Punto Mia (2000); the starter motor was turning fine, but the engine wouldn't ignite. I took
it to the garage, and within a day they had fixed it.
According to the invoice, they checked out an immobiliser issue and have recoded the keys and charged the battery.
Does anyone know why this might have happened (the receptionist didn't have a clue!)? The battery was definitely fully charged, because when I got the RAC out that was the first thing he checked. I don't understand why the keys would have become 'uncoded' in the first place? Anyone else had this problem?
Thanks, Ali

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