You can't use the card until you format it. Can be done on the PC, but if it is from a camera it's best to do it there.
A small disadvantage: you'll lose all data that's on it. If you don't have a backup, your might prefer to run a recovery program to get those off to a safe place before you format it.
http://forums.cnet.com/7723-7588_102-354911/lost-and-found-or-the-storage-forum-sticky/ mentions some programs you can try.
Kees
Please help me with TF card not formatted problem. When I connect my TF card to the PC, it shows "The TF card is not formatted. Do you want to format it now?" I've never met such problem, please help me!

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