Don't do any more writing to the card if you want to try to salvage what's on it.
Do you have a card reader? If so, can your computer see the card in the reader?
I've had very good experience with
Photo Rescue. You can download and try it to see what if anything it'll recover, and only have to pay the very modest fee if you want to do the actual recovery. Highly recommended.
Note also that there were some significant issues with some Lexar cards a while back, for which Lexar or Canon or both issued fixes. You might fall into that category.
Nill
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www.toulme.net