|I doubt you will be able to retrieve the lost ones. It could have reached the end of it's life (as already given) or have been caused by a glitch. Whatever, I would copy the good ones off the drive in case they go too.|
If you empty the drive by reformatting it, it might well store new files again. Whether you can rely on it any more is a different matter.
It is always best to have important files on more than one device. Any type of drive can fail without warning and flash drives are not really intended for backup storage, although they are improving.
We all live on a ball.