Put it in your computer. Open My Computer. Right click the memory card in the window, it should be listed under devices with removable storage. Select format. In the window that pops up, select fat32 as the file type, and leave the rest of the boxes to default. Click "quick format" then hit start. When it's done, right click the USB icon in the taskbar on the bottom right, and select to safely eject it. Now you can remove it from the PC and put it back in the phone.
If it still doesn't work, it means the card is corrupted and needs to be replaced. They're pretty cheap now, you can get an 8gb card for under $30.