Card might have gone bad. If you want to recover the data, don't follow the below instructions. You need to find recovery software or company to help with that.
If data is expendable, then continue reading.
If you're inserting it into an SD adapter, make sure the lock switch isn't on lock. If you're using it natively (miniSD card in miniSD slot), try formatting in Windows (not quick format).
If Windows doesn't format it, try the standard SD Association formatter (works with any SD card, regardless of maker).
https://www.sdcard.org/downloads/formatter_3/ (download links are obscure, look on the left panel).
If using the SDA app:
- Select the drive letter in question
- Open Options
- Select Full Format (Erase) and "format size adjustment" to ON.
- Then Format
If neither of these work, the card has likely gone bad. Either RMA or buy a new one...