Perhaps it missed the keep alive signal which would of course be an error.
BT have a dummy recording set up in the guide called viewing card activation I think its on Tuesday if I recall this is meant to keep the card active in case of non-use I would also assume errors in it missing the signal as well.
Set the box to record this on a series link just erase it from the box once the recording is done and hopefully if this does what it is meant to do your card would stay alive. The only other thing to do is perhaps ask the mods to look at your account to see if there is any other reason as to why it could have turned off i.e. Maybe there was a fault.
