I have a PowerShot Canon

Question

I have a PowerShot Canon S3IS and have encountered a problem whereby I have lost the complete content of my1 GBSD card. Whilst altering settings on the camera prior to an exposure a message appeared on the camera screen stating"memory card error" "card locked".This is the second occasion on which this has happened - but using a different memory card. The first time I reformatted the card and, of course, lost my all my exposures! This time I have not reformatted.Can you explain why this has happened and offer advice on how I might unlock the card and gain access which will allow me to download on to my Picasa file.I offer my thanks in anticipation of your assistance.