Recover bad floppy disk?

I have a floppy disk that go bad (can’t read). I wonder what can cause this problem and how to retrive the data from that floppy disk back? I also have a disk that bad sector at the beginning (the FAT area) so I can’t read disk at all, but when I using NDD to check I saw some used sector which is my files. anyone know how to get that data back or how to read that disk by bypass the bad sector?