Two causes that I've run across one is the slider is in the wrong spot just need to move it to unlocked or the card has become corrupted which means you need to format it. Usually happens when you go from one OS to another to load stuff or if you formatted it on a device that doesn't quite format it right.

To reformat put it in the card reader in your pc and under my computer find the card. Might show as a usb mass storage is you use a card reader or sdcard if you have it built in. Right click and format. You can sometimes copy your data off of the card first but most of the time you will see card or device not ready which means you won't be able to. So all you can do is reformat.