It worked before and I had tons of music on it, but something happened suddenly and it stopped working. I've formatted it many times. It says I don't have an SD card even in.
How are you formatting it? On the device or on a computer? Also, if you formatted it, then well your stuff on it is gone.
Depending on how many times you have reused the SD Card, the brand, etc., it is completely possible that it is toast and will need replacing.
I formatted it on my phone. I know the stuff is gone, but I just want the SD card to work. I haven't had it very long either. It says that I don't have any SD card in.
Well it could be the card is shot, especially if it is an off brand. It could also be that the slot in the phone is the problem. Have you tried another SD card and seen if that will work in the phone?