I asked a Vodafone support person why, and they gave me the predictable line "if you don't add money to it at least once a year its disabled".
Thing is I have added money on it within the year. I don't have exact dates but twice this year I topped up. 99% sure of it because I needed to use my mobile more for work. But of course now the element of doubt is in my head (even though I'm nearly certain).
The support team told me "sorry we can't re-activate SIM cards".
That is it.
Is there anything I can / should do? It seems like a load of old bullocks.