You may have inadvertently locked it if you have used it in an SD adapter (the ones that usually come with the card that allow you to use it in a standard size SD slot). There is a little slider on the side of the adapter that locks the card. If that is not the issue, the card may have become corrupted. I would try putting the card into a reader on a PC and see if the computer can access the card. If not, you will likely need to format the card. You will lose all your data, but you will hopefully be able to use the card again.
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