with age, thumb drives tend to just stop working. at 16gb i'd assume it is fairly old.
most likely it is just done and ready to be replaced. you can try sd card formatter and see if that does anything
www.sdcard.org
if it does not, then most likely it is just dead. the good news is they are dirt cheap right now. i bought a 128gb one not too long ago for about $18 and that was a samsung model!!