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Hello there, the first thing I would try is to download the free recovery programme called Recuva. With the card unplugged from your computer (I'm assuming this is a SD card) install the recovery programme.
Then plug in your card and run the recovery programme to see if you can see the deleted files still sitting on the card. If you can, and this is very important, you must recover them to your computer; NOT to your card!Once all files have been recovered from the card you can put them on a new card rather than overwriting your old card.Here is the Recuva programmeIf no joy, where did you buy your chart maps from please?