Did you format it in the phone?
Does it work in any other device?
Has it stopped working or has it never worked?
Have you tried the obvious things: rebooting the phone, taking the card out and putting it back in again?
If it is new, has never worked, you formatted it in the phone (or the phone can't format it) and it still doesn't work, get a refund because the card is broken. Don't waste your time trying to fix it, it's the seller's responsibility if the card is broken when they send it to you.
If you bought it from an online marketplace (eBay seller, Amazon marketplace seller, Alibaba, or anything like that) then if you can get a replacement test the card before using it: a lot of the cards sold in those places are "fakes", small cards repackaged and reprogrammed to pretend they are larger cards, and if you get one of those and use it sooner or later you will lose your data (basically once the amount you store on it exceeds the real capacity of the card). The simplest test is using a utility called h2testw which you can download to a computer - I don't know of anything equivalent you can do with a phone.